You mean killer allied CC units like ten GKTs with attached GK psycher in TDA? At 600 points for one unit, that probably guts ANY army you put it in. And, GKs being available to almost everyone, I can see it happening.
On the other hand, a careful, relatively frugal selection of allies can be very good for some lists, tau among them. Any psycher and any one credible-in-melee unit is a winner, but much more than that is probably "too much of a good thing."
Then, of course, there are armies that combine well to tell a story, like adding a splash of chaos to an IG list to make them traitor guard in more than just name. Adding just about anyone to orks with the twist that the detachment is actually in charge, they either hired the orks as mercs, or are fooling them into doing their dirty work. And so on. In those cases, the units displaced in the parent list and the replacing units in the detachment are probably not radically different in terms of tabletop performance.
The more I think about it, the more that I believe that most people who try some WAAC crazy powergaming stunts with the allies rules are going to end up doing themselves more harm than good. For example, say you think 'gee, I can put three IG russes in my blood angels list.' That's great, but you have to take a HQ and troop first, and the three tanks are a squadron, so it's not as cool as it sounds. 600 points later, you have three pieplates that have to maintain coherency and shoot at the same target. And think about what you'd have to drop in order to get those russes. And if you only take one, you pay 300 points for one tank plus a T3 HQ and a single squad of veteran IG. Yuck!