It might be a bit esoteric ... but the Photos that shipped after macOS Sierra introduced a clever new editing view built upon, I think, CoreImage so that it could do all the processing of levels, temperature, etc. extraordinarily fast — leveraging the hardware — for real-time changes as you dragged the sliders.
Unfortunately the retouch tool in Photos for cloning, fixing scratches, etc. took a huge step backward in performance. Like it became unusable.
So simply for dealing with old scanned family photos that have lots of scratches, tears, etc., I have stuck with Sierra Photos on my one machine.
When? Do you have any articles/videos showing the difference? I can’t find any.
The only major loss I’m aware of is when they left iPhotos and Aperture in the dust.