Sun/Moon is harder, yeah. In the past usually the AI during the main story sorta just chose moves at random and never attempted to employ any sort of strategy in the slightest. This isn't the case so much anymore, and that's why it's harder. Only other main story battles I recall generally being considered remotely difficult in past games are like Cynthia in DPPt, Blue in the Kanto games, Whitney, and Ghetsis, with at least a couple of those being for separate reasons though.
It's easiest to see in the Totem Pokemon battles. One of Totem Wishwashi's summons is Alomomola, a fat 'mon with Heal Pulse. Totem Lurantis can summon Trumbeak, making it difficult to use a Grass type to wall it or a Bug type to kill it, as well as Castform specifically equipped with Sunny Day to work with its Synthesis and Solar Blade (not to mention Totem Lurantis is holding a Power Herb too, they did that on purpose). The Salandits Totem Salazze summons deliberately use a Poison Gas (newly buffed to 90% accuracy) and Venoshock combo.
I did all these battles today, and despite carrying at least one poke with a type advantage in each, I nearly lost all of them. Yeah it's 2v1, but in past games the type advantage would've probably been enough to go "lol gg no re" and be done in a minute or two.
There's also the fact that the kahunas will use Z-Moves against you. Mega Evolution was used against the player in gen 6 too, but it was for like literally all of 3 battles very late in the main story--when you're far stronger due to better ability to level, have a much wider selection of hold items and Pokemon to make use of, less limited movepools, your Pokemon are likely in their final forms rather than the beginning or mid stages, etc. Here you have to contend with that power early on when you don't necessarily have these benefits, and I imagine more often too.
Whether or not it's too hard/still not hard enough, or is a good thing/bad thing is a different matter.
Or I just suck that's a possibility too
