To be honest, the difficulty is pretty sizeable if you are using in-game caught pokemon and not transferring already 6x31, potentially EVd stuff, maybe without your OT so you ALSO get the bonus experience.
Remember most bosses, totems and basically the entirety of TRR in USUM also are 6x31 with EVs, while you are going to be running with potentially mediocre nature, mediocre moveset and mediocre IV Pokemon caught in game.
If anything I'd think the opposite, USUM looks particularly brutal on someone who isn't actually accustomed to planning teams carefully.
I have no idea how someone inexperienced in pokemon is actually supposed to get past the fire, Electric and Ghost totem battles. They are beastly. Especially with the protect+torment tactic the former 2 use.