Im for the wackiest of alliances so long as you make an effort to make the following work: Fluff, Models/Representation, Deployment
the deployment part refers to how you actually organize your allies. Just because your Orks and Rines are allies doesnt mean they should setup close to each other and hold hands. I accept that sometimes in a war between several races in pitched battles warlords often accept the mose dire of alliances but they should still be represented as two separate forces engaging against the same opponent.With the exception of course with imperium allies I see most other alliances about two forces arriving on the battlefield seperately and striving for the same outcome, not necessarily bonding together cohesively.