A large part of writing optimization passes for LLVM consists of figuring out how methods/functions interact with the iterators in terms of validation and ownership. A lot of LLVM is undocumented unfortunately and it's not always clear how they will interact with the iterators. LLVM itself does not suffer from these problems much, but writing opt passes is definitely a huge pain.
My experience is that LLVM uses of iterator is very coherent with what to expect from C++, and I never had any issue on this aspect.
Iterator invalidation is pervasive in the STL containers, and as a C++ programmer you are supposed to know what type of container you're iterating on (in LLVM iterating over a basic block is a linked-list, iterator are not invalidated for instance).
