It's the size creep in heroes I'm concerned about, zombies not so much.
I have the original zombicide, whose figures sit quite happily alongside others. I didn't buy any of the other games until Invader. Some of the Invader heroes are okay, but others are head and shoulders above other figures.
I started to clean some for a Stargrave crew, as I reckon they'll work as a single crew, but won't really work in mixed crews.