They are pretty balanced even if they seemed over power at first. The main difference is the altform, though as I said they seem overpowered, but they are balanced.
Spire for example can climb walls in his alt form, the catch is he can't climb over ledges, or atleast the majoirty of them.
Weavel's alt form slipts his body in two where the lower half becomes a turrent, which stays still, that fires at the closest enemy and the other half is control by the player which you can walk around and attack with. You can reassemble at any time, the turrent will just dissapear, it doesn't matter where it is compared to you. The catch is when you split half the hp is slipt in half between you and the turrent. lets say you have 50hp and you split. If the turrent is destroyed when you reassmble(doesn't matter if it was destroyed) you'll onnly have 25hp which can classify you as a deadman.
So in the end multiplayer is balanced. Just use what works for you.