Here is nothing about realism, by the way. In real life the Marines would split themselves, without any "control" from the general. In the game, however, they would just stupidly stand and shoot, ignoring the fact that 2 Banelings can bust them all. This is something that I don't like about RTS genre in general, and about such fast-paced games as Starcraft in particular: a lot of time you spend on things that your army should do itself. Marines' split, Ghosts' snipes, Marauders' and Stalkers' hit and run, Sentries' shield... It would be great if they all would do it themselves, saving you a lot of time to do the management stuff: teching up, taking expansions, etc.
Now, it is bearable to have these things done by yourself in relatively slow-paced games, like Age of Empires, or even Dune series. But in Starcraft, it becomes really tedious. Instead of calm play with thinking and observing the battlefield, you have to spend a lot of time kiting these 2 rushing Zealots.
Jedi Knight 2 was much better balanced, but, of course, players found some exploits and destroyed the game with them.
The only way people could defeat 10 times bigger in size army is while defending castle. Before 1400 or so, when siege weapons became really powerful, it was literally impossible to take castle in any way other than by attrition, even if your army is 100 times bigger. You just couldn't get inside.
Well, in Age of Empires, if you have 2 times less units than your opponent, you are in trouble too. However, there it takes an awful lot of time to destroy half the enemy buildings, especially castles and towers, so the player with superior economy, actually, usually wins, and there is not much his/her opponent can do, because there are no cheesy units.
And, actually, EU3 was also quite easy, once you get a hang of it. Victoria II was far more complex. And Supreme Ruler: Cold War was more complex still.
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Actually, there is another great RTS many people have missed, I guess. It is Star Wars: Galactic Battlegrounds. Sure, it was just a copy of AOE2 engine with Star Wars sprites. But the campaigns were amazing, and there were a few new ideas as well: air units (too weak, but, still, interesting to play), Jedi who could both attack with lightsaber and convert. It felt like something between Starcraft and AOE, and also in Star Wars lore.
As a separate game, it is not that great as it is just a copy of AOE (very good copy, but still). But if you perceive it as an official AOE2 expansion, then this game really shines. At least, its single-player component.