In our case, it wasn't a matter of hitting a specific roadblock as much as it was past experience in performance-sensitive projects, and knowing that if we start with those crates we'll probably be fine, and if we don't, we'll probably switch to them eventually for some reason.

With crossbeam for example, you can hit roadblocks with std since their channels are MPSC, whereas crossbeam supports MPMC channels (and is faster than std in every meaningful measurement last I checked).