I'll just straight up say that I kept the exp.share on ever since Totem Lurantis because I do not like being stuck at one place for days and watch my pokemon repeatedly die. (I had Flareon, but it wasn't able to withstand Solar Blade) Besides, if overlevelling was going to be my only chance of winning the difficult battles, what's the point of stressing myself out by deliberately staying underlevelled. Keeping the exp.share off made the game hellish, but keeping it on didn't make it a walk in the park. It was still difficult even with overlevelled mons.

Almost every Totem battle required me to soft reset at least 3 times because they kept killing off my team. Biggest offenders being the Water Spider, Marowak (that Salazzle as its helper ._.), Lurantis, Togedemaru, Mimikyu, and Ribombee.

I even lost a couple of rival battles because my pokemon just kept flinching when Hau's Torracat used Bite. Guzma's Vikavolt packing Flash Cannon also caught me off guard.

Necrozma Dusk and Ultra Necrozma were, surprisingly, the only battles I found quite tolerable, even easy. Biggest reason being that they never healed themselves, unlike Yveltal who kept sucking the life outta my mons for its own HP, or Primal Groudon who'd spam Rest. So one successful Toxic via Salazzle on both of them, and a few health restoring items later, the game was set. And mind, my Salazzle, despite being the weakest member of my team, loved me so much that she survived a supposedly guaranteed OHKO with 1 HP.

The E4 were pretty exciting. Molayne's use of Prankster Klefki really surprised me. So many spikes XD.

So, thus, my opinion of the difficulty of USUM:
Totem battles>>Trainer battles>>>Necrozma/Ultra Necrozma