Well, the concept of Mega Evolution is from Gamefreak, so the anime just had to adapt it obviously. I personally love Mega Evolutions. Also, Ash Greninja is again Gamefreak's fault for not giving Kalos starters actual Mega Evolution, and since the anime always needs to promote the current gen PokÃ©mon, they had to give a special Greninja to Ash that could take on Mega Sceptile and Mega Charizard X. If not, Greninja would have stood no chance against them. Personally, I loved the idea and concept of Ash Greninja. It made it feel very personal between Ash and it, as it meant that Ash was able to tap into that hidden potential and bring the best out of Greninja, and to the next level. Their arc was superb, especially their struggles in trying to master the form. And it brought us XYZ028 (Winding Woods), which is one of my favorite episodes of all time.
I don't want to comment on BW. I hated it from episode 1 itself, when they decided to reset Ash for no good reason, and make him worse than even his rookie OS self. I hated the companions as well, especially Iris. However, the early battle tournaments were pretty nice idea wise.
SM has been mixed for me, though slightly more negative than positive (I'd say 45% positive and 55% negative). I hate the school concept, since everyone is stuck on Melemele Island and can't travel. I personally don't care for most of the characters except Kiawe and recently, Gladion. And Ash, well I really dislike how much of a joke he is. Always running into unfunny slapstick humour and acting like a buffoon most of the time (thankfully he's still good at battling). As for less filler, yeah by sheer number, it may feel that way, but the problem with this SOL style is that most of these episodes just feel like filler, with maybe something important happening towards the end. So they still feel like boring, but important episodes.