[Zope] ZClasses deprecation and upgrades

I am wondering about is the nature of the problems with ZClasses that led to their deprecation. If I move a 2.5.1 installation to 2.11, are my several existing ZClass-based products going to break in any known way I should be aware of? Should I consider a last-known-good upgrade target instead? Not really looking for ironclad assurances so much as general experience here.