AFAIK the biggest issue is the nondeterminism ("inconsistency"), especially in languages which have a weaker type system and make upgrading easy (which is the

AFAIK the biggest issue is the nondeterminism ("inconsistency"), especially in languages which have a weaker type system and make upgrading easy (which is the case of both Go and Java) - you need to remember to check the result for nil/null, but the type system won't tell you - if you upgrade twice in a row, you can have both fail, both succeed, or the first one succeed and the second one fail It can also be tricky to use them as building blocks for weak collections, if you want to avoid unbounded growth.