Orcs

As far as rank and file troops, Sauron's forces are mainly orcs. There is some evidence suggesting Tolkien's depiction of orcs was based on ideas common to when he was writing regarding racial degeneration and mental disability. He went as far as to say "they are ... in fact degraded and repulsive versions of the least lovely Mongol-types". Dr. Dimitra Fimi points out that Down Syndrome at the time was called "Mongolian idiocy" or "Mongolism".   Dimitra., Fimi, (2009). Tolkien, race, and cultural history : from fairies to Hobbits. New York: Palgrave Macmillan. ISBN 0230219519. OCLC 222251097.   Carpenter, Humphrey, ed. (1981), The Letters of J. R. R. Tolkien, Boston: Houghton Mifflin, ISBN 0-395-31555-7   Their primary use is as the normal troops. As noted in the sections dealing with Sauron's military doctrine, they more specifically are meant as cheap, disposable cannon fodder. This is a solid role for them. Apparently they can be bred rapidly and have relatively low upkeep costs compared to say a troll. Trolls are solid shock troops but sustaining a large army of them would be extremely costly. Plus they make obvious targets. So having a group of more common soldiers to protect the trolls and exploit the breakthroughs they produce would be a huge benefit. Not to mention that higher numbers allow for flexibility and higher reserves.   Also, the Ring Wraiths are great shock troops too. But they are totally inadequate for occupations or large battles. They can do significant damage and scare the enemy. But they are not numerous enough to be able to conquer and hold whole areas. Essentially, the second they win somewhere, they need to be sent elsewhere. Which then opens their enemies to slip back into the "conquered" areas. Or a simple diversion can be used to pull them away from the main battle. Their enemy can can hide in their forts too. Essentially orcs are needed to consolidate gains even if they are not the main force. Which then means they certainly fill a vital function that cannot be replicated with Mordor's other assets.   So, perhaps I am a little hard on the orcs in other sections. It appears that their defeats come from a lack of training to hone their innate talents. Or they lack good equipment or leadership. But they are strong enough and possess enough of a ferocious fighting spirit that they make pretty decent soldiers. Should Sauron use better tactics and actually care for his troops, he would do better in battle. But because of the apparent speed in which one can breed an entire army, it can be assumed that massive losses from poor tactics would not be as fatal as it would be for other species.   There appears to be two races who are better in this role than orcs. The first are dwarves. They have many of the same advantages. Especially regarding endurance and strength. They just don't have the weakness to the sun. The other is the uruk-hai. These are literally orcs who were bred to eliminate many of the weaknesses of the orcs themselves.   They do suffer from one major downside. That is they seem rather easy to kill. Granted, they are quite good at killing. But nothing stops the enemy from killing the orc before they get close. This is especially the case when one realizes that orcs rely on some of the weakest bows in Middle Earth and brute force melee attacks. So any enemy that has decent ranged capabilities can do major damage to an orc army before the orcs even get into range. This is part of the reason that huge numbers are needed.   But once they do get in close enough for melee, it appears not too many people can resist. Through a combination of physicality and numbers they dominate at that range. Brute force allows them to hammer through defenses of enemy troops. This allows an army based on orcs to compensate for the apparent weaknesses. Essentially a single hit from an orc can knock a soldier over, which allows a more leisurely penetration of armor.   In the mental games essential for good monsters, they are a mixed bag. Similar to their physical abilities. They do inspire terror in their enemies. First, their appearance is considered revolting to most of the free people. Their use of rotting flesh and so forth for their clothing leads them to smell rather bad. Plus their weapons are rather crude. Both of these compound their reputations for cruelty and carnage. Yeah, if one wants rank and file troops that can intimidate their enemies, I suspect that only the uruk-hai would be better.   They are also quite well suited for small unit operations against civilian targets. So they would be good as an occupying force too. They can also be an effective weapon of terror against the civilian areas. The fact that they are already suited for small unit raids that use excessive violence, they would be perfect for terror attacks against enemy villages. When the main invasion force gets going, it makes perfect sense to use small bands of orcs to attack soft targets similar to how the orcs and wild men operated against Rohan. The idea is to steal supplies and deny the leftovers to the defenders. Then, burning and killing everything else intentionally to spread terror.   Where they are less effective is in securing the obedience of allies. Though this is somewhat irrelevant. Their position is at the absolute bottom of the food chain as far as Sauron is controlled. All other command and control measures are meant to control orcs. There are a few points where orc captains pop up that use the means of coercion against other orcs and occasionally a troll or two. But at no point do they impose their will over Ring Wraiths for example. Essentially they are there to be controlled instead of doing the controlling. Which means they do not need to be as good at commanding obedience.   For Mordor's coalition partners, the orcs are somewhat effective. Their ability to hit hard and fast against soft targets would be quite effective in doing damage against the plantation economy of the Corsairs. The small troop densities and poor equipment of the Southrons would also be easy prey for orcs. Simply let a bunch of independent raiding parties raid Harad and they would inflict unacceptable losses. The Easterlings though would not be scared by them. In fact, I think they would relish the opportunity to fight against such a warrior race.   Their intelligence is also quite adequate. They are not amazing. But they are smart enough to invent new torture methods, follow orders and make basic weapons. But they are not smart enough to figure out how to effectively cooperate on their own. Well, not smart enough or too hateful of everything. Either way, they can't see the value of cooperation for the sake of mutual self interest. Which makes rebellions difficult.   And yes, they are quite well battle tested. Insanely so. We see them put through extreme conditions without breaking and fighting to the end. At least two defeats of orc based armies took a decapitation strike that "killed" their leader. The other major one was the defeat of Melkor. In that war, the orcs almost won before an intervention by the Valar. When the gods of a society are required to defeat an army, probably said army is pretty decent. These battles did demonstrate their imperfections though. Which then led to the creation of the Uruk-hai.   Now, there is one excellent fatal flaw for orcs. This is their vulnerability to sunlight. Depending on which group of orcs and whether we are talking books or movies, the power of the sun varies. Sometimes, it outright kills them. Sometimes it appears they are simply afraid of it. Other times their long periods of time underground led to eyes evolved to see in dim light. Which means daylight would hurt their eyes. Uruk-hai apparently did not fear the sun, but it still burned their skin. Basically, sunlight does not lead to them having a good day.   This led to one of the most brilliant moves on the part of Sauron - the Dawnless Day. Technically it lasted from March 10, 3019 to March 15, 3019. Black fumes, most likely from Mt. Doom blotted out the sun over much of Gondor and part of Rohan. This would naturally have profound demoralizing effects on the defenders. Plus, the fumes would probably be effective chemical warfare. Perhaps not chlorine or mustard gas level bad, but it would certainly take a toll.   Lastly, the orcs get protection from the sun. This allows faster movement as the armies do not need to stop for camp each day. It also serves as a powerful command and control feature. Essentially, should Sauron fall, the army effectively falls with him. Or if the troops get rebellious, the clouds can disappear as punishment. Think of this as a form of emergency kill switch for an entire army or organic beings. So I would say that the sunlight weakness would be profoundly good flaw.   Are they a threat to their masters? I would say no. They certainly would be no match for the Ring Wraiths. The Dark Lords all appear too strong willed and terrifying for orcs to resist. Nor are they able to cooperate enough to pull off rebellions. Then there is the whole ability to control the sun. Lastly, by compelling them into the army, the dark lords can give them greater ability to act out their violent urges. Basically resistance is clearly too high, odds too low and rewards for cooperation too good.