Well I don't think it would be so easy, but compared to an RPG definately not.

However the time to properly balance the game would depend on the amount of play testing you are able to get feedback on. But yeah, the more unique units each race/faction and the total number of races/factions you have would of course increase the extra balancing required.

Overall, I think it would depend on the complexity of the RTS in question. So yes, it could take quite a while to get all of that in order, but no where near as long as it would take to put together an RPG of similar complexity. If you could even compare the two that way.