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Could be the wrong spot, but It seems with all imperial armies being battle brothers and the current ally rules you should be able to combine things to make a pretty much unbeatable list for almost any other army that really can't be hard or soft countered. Just looking over the codices I see a lot of power from special rules being able to be conferred to units who were never intended to have them. Why aren't these lists totally dominating? What am I miss interpreting?

You're not misinterpreting anything. It is totally allowed. And this is the wrong forum, as it's not a rules question but rather a tournament question.

As to why they aren't winning all the time? Well first, models still cost points - so it's not like you can just cherry-pick single models to make awesome, you need to include whole squads or pay for at least one other unit to have it. Also, many tournaments restrict the number of sources you can bring, so if it's a max of 3 sources, then you couldn't take from Grey Knights AND Imperial Knights AND Space Marine Chapter 1 AND Space Marine Chapter 2, as that would be 4 sources. Finally, even with all the Imperial stuff out there, there's still a lot of stuff that some Xenos or Chaos stuff that can do things better (like summoning daemons, or having Wave Serpents). So there's a bunch of reasons, no one of which is solely responsible for the current meta.