One difficulty is that some of the armies (particularly in 5th) were really weak. The Empire comes to mind. The kind of army that can win in 5th is basically illegal in 6th.
There's also the issue of new or changed lists - Undead being split into Vampire Counts and Tomb Kings, for example.
As winter settles in, I'm looking to do some gaming (using Conqueror, of course) and while I have "army lists" in the book, there's nothing to stop people from mixing and matching. Even though I've had this system out for a couple of years, it still throws me off a little when it happens.
To give an example, it's perfectly legal to create an army concept based around knights of evil (chaos knights) with an orc retinue and dragonkin (lizardmen) auxilliaries. Whatever your campaign envisions is fine because the units are modular (a working point system! What a concept!).
Anyhow, it's just funny how much I've internalized the GW concept of army lists all these years later.