> but assuming we're speaking about their implementations in the industry (rather elaborate tracing GCs; rather crude refcounting GCs)

That might be because a tracing GC needs to be rather elaborate to get good performance and avoid bad behaviors.

> When exactly that happens and how much work there is to do (and sometimes, what the fragmentation effect on heap is) are local properties

It being a local property is a good thing. It means it won't affect or be affected by how the rest of the program is behaving.