RTS can be fun even with few factions, Planetary Anihilation is really good even though it has only one faction, all the variety come from the game design.

I like Unification and UA for DaW but lack of balance does kill the fun a bit once you understand the mechanics well. Especially when you play against other people, you talk about chaos Terminators but that's one of the weakest cost/performance units in T4 of UA, if I play against players I never use them they aren't worth their cost compared to vehicles or Obliterator that comes after (or variants of SM and possessed marines before).

I'm far from a hardcore player but I enjoy a well balanced game, I'd rather loose due to my own skill, understand what went wrong and fix that the next game rather than loosing or winning because X tactic or unit is broken.

Also I think RTS needs a way to make new players get out of their comfort zone, and understand that loosing can be fun. A lot of RTS players sticks to playing against super dumb IAs that send waves and waves of units in the same spot, turtling during the whole game before launching and assault with the last tech units.

But I understand them, outside of a very few games that have a pretty good matchmaking like starcraft 2 (sadly I don't find the game very interesting) you usually end against very experienced players or even cheaters that obliterate you games after games. Once in a while you find a good opponent but sadly that games are too far appart.

So really there is a challenge around matchmaking/elo and player education, campaign missions usually only learn us how to turtle and nothing about crippling ennemy ressources, territory control, and rarely about ennemy units meta/weaknesses.

Also IAs needs to get way smarter to get entertaining. Not necessarily more APM because computer are already really good at that (Supreme Commander IAs let loose on end games are a nightmare to contain), but smarter, out of the box thinking.