When can a Smogon format ban a move or ability? Is it only possible when it has multiple abusers that make it broken? I've heard people say that tiering policy can ban it if it's "inherently uncompetitive" (eg. it's luck based or prevents switching) and not broken because its abuser has good stats or other good moves.
See
this discussion (which was AFAIK never officially written down into any policy guideline, but has been de-facto adopted as policy).
Smogon prefers to ban Pokemon to other metagame elements
where possible; when something is broken, it is typically sufficient to ban the broken Pokemon, in which case doing so is preferred. However, in the cases of something being uncompetitive or unhealthy, lowering the power level by banning the strongest abuser doesn't necessarily remove the problematic element. Instead, in such circumstances, Smogon prefers to ban the
least impactful element. Your examples of luck and switch prevention are relevant, but another example is Baton Pass; while banning the best abusers lowers the
power of Baton Pass, it doesn't remove the cheesiness and unhealthy nature of the strategy. And while Smogon could just ban every Pokemon that learns Baton Pass, banning Baton Pass as a move is clearly less impactful.
The exact specifics have come under discussion recently, e.g. with
Tatsugiri in DOU being a particularly thorny case — the Dondozo + Commander combo was deemed
broken, but Dondozo is not inherently broken, Commander is. Tatsugiri, as the sole owner of Commander, was banned, but it is slightly concerning from a policy perspective that the characteristics of Tatsugiri itself are totally orthogonal to the reason for the ban (if Magikarp or Luvdisc got Commander instead of Tatsugiri, it'd still be broken). Still, this decision affirms the precedent that, when
possible to accomplish tiering policy with a Pokemon ban, that is preferred (so a Tatsugiri ban was preferred to a Commander ban since it "did the job").
Tera is another recent case where this has come up, with a lot of discussion revolving around whether banning x number of top abusers is justified by policy rather than just banning the mechanic, but to be honest I think most people in that conversation care less about the policy implications and more about their personal thoughts on whether Tera is healthy or not.