Think about mega mawile, it just needs a bit of support (removal of fast willowisp and chip damage on bulky pokes) to sweep. Other sweepers, like mega scizor, all are much easier to check - most fires can take a bullet punch/knock off, so it needs a more support to be able to sweep. Most of those pokes you have mentioned do have counters and most prepared teams can stop a sweep from them; scizor- faster fire types, cm clef - bisharp/scizor/taunt users, dd's - prankster twave/scarf pokes (garchomp eg), bisharp - anything faster resists sucker punch/quagsire. The difference with mawile is that due to its power it only needs 1 boost to easily tear through a team. Mawile doesnt require as much skill to use as other set up sweepers and doesnt need as much support, although a bad player wont be able to utilise it well enough for a sweep.
Baton pass was an exception - it wasnt one mon, but a team that could deal with every poke in the meta and sweep with lack of skill.
It is hard for a mon to be an instant win button, because most pokes have counters (arguably not xzard/lando-i). The reason why the mon is broken is that they can find ways round counter (ice punch/eq mega luke) and some of the counters are obscure (bw2 lando-i ppl used arcticuno) or sometimes it requires a counter on a team (rotom-w shot up in usage bcus of lando-i.
Remember, no one should let a poke get a free turn of set up (definitely not 2) - the reason why mawile is so good is that it easily finds pokes to set up, that it either walls (latwins) or are scared of getting ko'd.
