1) Mainly because Snova decks take a long time to get set up after the Snova nerf (Singularities). You lose 1-3 turns, which is all a monorush needs. As for why it's monorushes specifically: they're the most consistent rushes in Arena, partially due to how the AI works and partially because of stabler quanta. It's especially bad if your controlbow focuses on Wings for defence, as you'll always take at least one hit from Dragons and the like.
2) MA SoW isn't actually slow at all. If you ran into a few that were, they were badly designed. Either way, yes, they're extremely powerful. Almost none of the best Silver grinders can beat them consistently, because even if you deal with their defense (Pulverizer for Dims or Cloak for 12 frickin' Lightnings), there's no way to stop their offense. A Mirror Shield in your controlbow can go a long way towards giving you a better chance thanks to the AI being dumb, but you might still lose to normal physical damage that they pack. As for Bronze, since they have no Dexterity, a Mirror Shield would do you wonders. As long as you have 1-2 Electrum Hourglasses to make sure you draw everything you need.