Agree with most of this except for "and many specialized allocators have their own security mitigation build in." This is generally not true in my experience [1] when it comes to unlink checks, guard pages and the like. Often a generic homegrown allocator will call mmap/VirtualAlloc and chunk the page(s) up into a doubly linked list or simple arena allocator.

Sometimes a default generic allocator will have design properties about it that make exploiting certain bug classes difficult (e.g. WebKits RenderArena and Use-After-Free). There are exceptions of course, see Chromes Partition Alloc for one example.

Heap hardening/exploit mitigations are hard to get right. Ptmalloc has done a decent job save for 1 or 2 techniques lost to time. As a result modern exploits target application specific data (C++ object vtables, function pointers, etc) and not heap meta data.

[1] I wrote this blog post and I audit code for a living