The predictable runtime performance is also a myth, because they never take into account the use of NUMA memory, lock contention, possible stack overflow and stop the world in the case of cascaded deletions in naive implementations.
This is proportional to the data structure being deleted.
Unless you use techniques to move the deletion into background threads, e.g. C++/WinRT with COM AddRef/Release, the thread will be "blocked" doing busy work cleaning all those nodes, running the cleanup code (destructors, deinit, whatever), node after node.
I'm not sure most GC implementations worry about the rest neither (as by the several complaints we see going around)
(makes me wonder who's buying those - things like Azul, etc)
There is nothing "special GC" about it, the failure is to assume there is only one way to implement GC algorithms, as if there is only one way to implement hash tables, tree re-balancing algorithms, .... and then place all languages into the same bucket.
Then we have the modern times with AI driven code generation, where no one cares how their agents are actually doing the work, with what kinds of resource management approaches.