The other way where managed memory is a problem is when it fails to deliver the performance of the usual memory allocator, forces "stop the world" events simila

The other way where managed memory is a problem is when it fails to deliver the performance of the usual memory allocator, forces "stop the world" events similar to mentioned defragmentation... And you're back to square one with the added complexity of having to work around the memory allocator in place. Java and its GC implementations come to mind.