The point is that it's designed to work with native code in a language that wasn't designed to use garbage collection. That's going to result in a semantic gap

The point is that it's designed to work with native code in a language that wasn't designed to use garbage collection. That's going to result in a semantic gap no matter what, and it invariably does. Every system I've ever seen using Boehm GC has issues as a result, except in certain special circumstances where the limitations are mitigated by the nature of the system. That's not an indictment of the GC itself, it's simply a consequence of trying to shoehorn automatic GC into a language that doesn't support it.