[Zope] ZClasses deprecation and upgrades

[Behrens Matt - Grand Rapids]

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.