I honestly don't know why people found XY so disappointing. I personally enjoyed it a lot more than S/M, which I found to be downright infuriating.
So I guess I'll say then that X/Y to S/M felt like a downgrade. To me at least.
I was going to suggest that RSE and USUM probably exist in the same dimension, until I remembered the whole Anabel plot. So don't really know what to tell you then.
Honestly the whole "alternate timeline / dimension" thing just over complicates everything and I wish it weren't a thing.