Say what you will about gen VI, but it at least has the distinction of being the last "complete" generation in a sense. All the Pokemon were there including their dex entries, all the moves were intact, the online was better than it had been ever before or (sadly) since, it added a brand new battle mechanic in Megas, it still had all the extra battle modes gen V introduced (namely Triple and Rotation Battles) as well as adding another in Sky Battles (as poorly implemented as it was) and ORAS even brought back a couple lost game modes with the return of Contests and Secret Bases.
Then you get to VII and things start to get a little iffy. Contests, Secret Bases, Triple, Rotation and Sky Battles were all axed, the wonderful PSS was replaced by the horrendous Festival Plaza, the dex didn't have entries mons not native to the region, and Megas were pseudo phased out and relegated to post-game only. About the only new major mechanic added was Z-moves.
And finally shit really hit the fan with VIII where (thus far) all the above cuts still remained, Megas and Z-moves were axed, over half the Pokemon roster and quite a sizable chunk of moves were deleted from existence, the online was made even worse to the point where you don't even have a friend's list and have to use an archaic code system on top of the GTS disappearing (a staple that has been there for over a decade mind you) along with the host of other general quality grievances that are too numerous to list.
The Pokemon series has always had this annoying habit of adding and removing ideas and innovations between generations and even individual installments, but it's downright shocking just how much content was slashed within one single system jump, one that was supposed to be a momentous occasion that many fans have waited countless years to see.