Byleth did teach me a very valuable lesson about pre-orders and hype that I learn from to this day, however if we're just talking about how that affected my approach to FP2, it didn't cost me any more or less, since the only character I disliked enough in that pass enough to not buy was Pyra/Mythra. Even Min-Min, who I thought was lame before and after her reveal/showcase, I bought, because for some reason I thought I was gonna go all-in and become one of her top players or something. FP2 in general was very much a redemption arc from Byleth I think, each inclusion served a purpose - even the one I hated. Mostly due to just I thought this was gonna be Eggman's moment and it was not :/, but also because of the lame, awkward trailer and the recentness of Xenoblade Chronicles 2.
I did actually try out Pyra/Mythra a couple weeks ago at my school's videogame club on a friend's Switch. To be honest, Mythra was really lame, just like how she is to fight. I'm not good at combo-heavy characters but at least they have fun moves for people who don't really get them, like Luigi's Green Missile or Pika's and Pichu's down b. But Mythra just feels like she was pulled out of a super technical platform fighter in an alternate universe where Namco turned Nintendo down on Smash and then capitalized on the platfighter trend years later.
Although, Pyra was actually pretty fun - also, like she is to fight. Big, sweeping moves that do a ton of damage and knockback, a cool shield breaker, and a recovery that makes you feel like a god if you hit with it. She actually does feel like a Smash character, makes me wish the Challenger Pack was just her and maybe Rex for a few attacks, like a lot of her supporters suggested.
But overall, I can see why people like them from a gameplay stance, assuming you're really cracked with Mythra's schmovement and quick attacks, because it only takes a second to switch to the character who can KO with any move. And both the "clones with different specials and stats but same normals" and "clones as a stance fighter" concepts are really cool I think, and should be used in a future game for sure.