[obl]

Unfortunately that's only superficially true. You have to care about GC interop, unwind & debug dwarf data, implementing part of the ABI in the frontend, and of course the occasional bugs when you exercise code path that clang does not.

Because of this, enabling a new backend is a substantial amount of work even if the instruction selection/generation is indeed taken care of for you.

[piokuc]

Sounds like you have some practical experience with that. What frontend was it? Are these problems frontend/language independent?