I've personally stumbled upon two - obscure hardware and GUIs.

It's really difficult to evaluate a hardware product with only a C(++) SDK using only Rust. There are of course ways around this but those are a significant amount of effort.

In terms of GUIs - there's simply no "Qt-Rust" that would work in a Rust-y way without causing massive headaches. Handling all that state is incredibly difficult in Rust, at least for me (but I recall others describing why as well).