20x rule is on safe side where most of time set mine will show profit. Though common spot where it could still be bad play is e.g. BU open raise and calling from SB. Reasoning is that small pocket has very weak postflop playability. Combined with being versus wide range oop will make getting good enough implied value when hitting set hard.
When spot is multiway or we play versus tighter range our implies odds will increase. So if we open from let's say mid/early position and someone 3bets we will likely be versus tight range. Villain will often have overpair or tptk type holdings when we happen to hit set. So chance for stack off will be higher.
Also it's good to remember that sometimes when we hit set opponent will have bigger set, flopped flush or straight.
These general 15x or 20x rules aim to take account those variables. But depending of exact spot and opponent(s) there is space to bend them somewhat.