To stay on the focus of OU, I've really noticed that most of the meta game this gen revolves around ghost and dark types. Is that just because the ghost and dark types this gen are that good or is there something inherent about those types that are giving them an advantage?
Well, Game Freak made 4 part-Dark (sub-)legendaries this generation, of course they're gonna be pushed. (Wo-Chien falling to UU is mainly a matter of its crummy defensive typing, IMO.)
And then Game Freak made a whole bunch of 570-590 BST mons, 4 of which are part Ghost or Dark. We booted Flutter Mane to Ubers, and Iron Jugulis and Brute Bonnet are floundering, but rolling the high BST dice pays off and Game Freak struck it rich with Roaring Moon.
And then Game Freak gave Rage Fist to a Ghost-type mon with decent attack and good bulk, so it can actually take advantage of that breakable attack. (Game Freak also gave Last Respects to at least one Ghost-type mon with non-bunk attack, which got it booted to Ubers.)
And then Game Freak decided that this odd-numbered generation's Magikarp parallel's evolution was going to be worth it being gated to the late-game (seriously, look at Gimmighoul's paltry 2-move level-up movepool), so they gave that evolution a swell defensive typing, a nutty ability, a powerful unique STAB move, and just enough bulk to check what it should (mainly Fairies) and some things it shouldn't. (Fun fact: pre-release leakers indicated that Make It Rain was originally going to be 140-150 BP with no drawbacks, then the test hammer hit. Said leakers also admitted that Good as Gold was overpowered.)
And then Game Freak thought that Bisharp needed an evolution for some reason.
And then Game Freak's endless dice rolling with starters actually gave us 3 viable ones for a change - one of them being a Dark-type with (nerfed) Protean and another one being a Ghost-type with Unaware, Will-O-Wisp, and actual bulk in a meta with more physical attackers than special ones.
So you can definitely argue that Gen 9's Ghost- and Dark-types are pushed.
One thing affecting Dark- and Ghost-types' viability this gen that came from a previous generation, though, is that
Dragapult has not been snapped. Partially due to Pult's frailty, Sucker Punchers are up as a result. Choice Scarfers that outspeed Specs/Boots/Band Pult and hit it super effectively get noticeably better as a result. (I suspect Pult is part of why Choice Scarf Heatran didn't get much traction last generation - Scarf Tran cannot outspeed Specs Pult.) Note that Ghosts and Darks have super effective STAB against Pult, and even Scarf Gholdengo outspeeds Specs Pult.
You'd think Iron Valiant would be making out like a bandit in this environment, and it kinda is, but it really doesn't like taking a neutral hit - and the Dark-types can provide plenty of neutral hits to the current highest-tiered Fairy in the meta.
Overall, I'd say that Ghost- and Dark-types being good in this meta is partially - perhaps mainly? - because they're pushed, but other meta considerations such as Dragapult (and Espathra) are pushing them even higher.