Alliances don't have to be beneficial that much....there are plenty of ways to nerf them.
When alliance attacks it needs to gather armies since there are some problems when mixing different armies they need to pay higher maintenance for that army. Also that kind of army might suffer some setbacks while on the march...
Secondly that kind of army would be spotted early so you might have a chance of attacking them on the way. Might even have time to set traps ahead of them...
If attack is successful large part of the loot gets "lost".
The army is not as efficient. Because lets say the elven archers don't know how to fight effectively in combination with human knights.
And at the end you need to pay a tribute to the alliance you are a part of. The bigger the alliance higher the tribute and stuff like that.
(mind you i didn't play the game so all this stuff is my guess of balancing)