For one, are you looking for MP or SP or both? SP it won't be as noticeable. The people who notice the most imbalance are of course the ones who spend the most time on it, i.e. the pros. I would not call it glaringly broken. Perhaps now with the patch 1.03 the most obvious is the jumping Orkies, and the devs are going to work on that. If balance is your only concern, I would say get Rebellion, it'll be worth it if you like the rest of what you see.

Note: I myself do not have Rebellion, however it is not "imbalance" keeping me from it but friends on LAN. We use the older Trinity and I don't want to get Rebellion unless they do.

It depends on whether or not you play MP.  If you play MP, you'll notice them as you become more and more skilled.

Balance between units at this point is actually some of the best its been in the history of the game.  The Advent are underpowered and the Vasari are overpowered.  As I said though, unless you jump into competitive multiplayer, this is pretty much a non-issue.

If you're worried about advent being UP, play the rebels.  The advent Loyals kind of suck right now, but the rebels are much better off.

And hey, while nalance is still kind of poor right now, it's much better then it was prior to the 1.03 patch, so we're making progress.

I cannot honestly say that this game is balanced. However, the more important question is "is the game fun" the answer in my oppinion is yes. A large portion of fun is variety if you want true balance you can have it ...... as long as everyone wants to play the same faction on a perfectly symmetrical map but that is not fun. Therefore having perfect balance is never going to be achieveable.

Bottom line is it is actually not half bad give a skilled player any faction and they will rofl stomp a mediocre player even exploiting the biggest imbalances.