As I stated in a previous thread, Johto, DP and XY all feel like 'refinement series', they take what made the last couple of series good, streamline the whole thing and put greater emphasis on a more focused and linear storyline, while the other series have way more attempts at experimenting. I have my beliefs that Gen 8 will likely be the same as the former series, looking at what worked in SM and developing it while dialling back on everything else for something more consistent, for better or worse.
I think a recurring problem with these three series is that they often push for their main focus to be the draw appeal, and everything else tends to be a heavy last priority. This means the draw appeal is heavily reliant on them making the centrepiece material good, while everything else is padding at best, and usually very lethargic padding (there's a reason these three are known for their TERRIBLE fillers). As said before the others experiment more. That means a lot more flops and inconsistencies in their main material, but things can always change up and improve. A lacking area in SM can abruptly get a tune up and suddenly be given a lot better handling, even during the very later points of the series, though another can just as easily nose dive in quality, while with DP and XY, though more consistent with what they DID want to improve, usually when something was faltering, you knew it wasn't really going to get any better, it just wasn't part of the big picture they were focusing on in those cases. The problem with XY was that fans were banking on a big formula breaker, and that series was built on a formula.