It's likely that some things seen as too strong on lower ranks are less so on higher, where the meta is different and where people're better at playing around stuff.
I mean.... one faction can fill half their deck with thinning tools able to dump tempo and the other half with cards able to exceed the provision cost reliably. Strong tall counters, engine killers, a decent lock, damage engines, a high provision number, versatility, and the list goes on. The other gets to spam unconditional engines anywhere on the board, unconditional tempo anywhere on the board, has access to decent thin options (which, by the way, also provide the temps), and can arbitrarily stuff random cards in it's builds and still win. I'll allow you to guess which two I'm talking about.
Are other factions competitive? Absolutely. Can they beat the golden children? Absolutely. Are they as strong, in as many different ways? Nope. Every faction has a way to be competitive. The fact remains if you asked me to play one faction to grant the best chance to win, without any knowledge of the match-ups ahead of time, it's SK or MS. I'd probably err toward SK just because it's very strong and can nearly always find the cards it needs. I still think it's a toss up between these two as to who runs the playground.