> Can't do GC unless you go all-in.

It can be done, especially with a safe non-GC language that can meaningfully guarantee it won't corrupt GC metadata or break its invariants. You only have real issues (and then only wrt. excess overhead, not unsoundness) with pervasive mutual references between the GC and non-GC parts of the program. You do need to promote GC pointers to a root anytime that non-GC code has direct access to them, and add finalizers to GC objects that may need to drop/run destructors on non-GC data.