I wasn't sure about Satoshi-Gekkouga at first (the design took some time to grow on me), but I like how the anime staff has been handling it. This is the first thing in years to actually give Satoshi some character development, and that's made
XY&Z more enjoyable to watch. One of my stated problems with
XY, as
Satoshi said above, is that Satoshi hasn't had much to focus on besides the next Gym battle. Satoshi-Gekkouga gives him a goal outside of that, much like Shinji in did for him back in DP.
And then there's it's power: fighting Alan's Mega Lizardon, able to go head-to-head with Carne's Mega Sirknight and not lose instantly, fighting (and, presumably, defeating) Urup's Mega Yukinooh in the next episode. The anime usually likes to play "David and Goliath" with Satoshi against tough opponents, so it's nice to see him finally capable of fighting them head on instead of being the clear underdog. I just love that Satoshi has a truly powerful Pokémon this time around, amidst a fully-evolved team (well, excepting Pikachu).
Unless they're building up to something with this for Sun and Moon, I can't see why they didn't just give Ash a Mega Evolution either, and had him struggle to control that. This would have worked just as well with his Charizard, and whilst I'm sick of seeing that thing, it would have served as a nice counterpoint to Alain. Or his Sceptile to make his rivalry with Sawyer more interesting
My personal theory is that the writers wanted Satoshi to have a Mega, but didn't want to break the tradition of Satoshi advertising Pokémon exclusive to that generation. And, aside from Diance (a legendary), there aren't any sixth-generation Pokémon with a Mega Evolution. So this is the compromise. Bringing back Lizardon or Jukain, even as Mega Evolutions, undermines that. And for what it's worth, I don't see this storyline dramatically changing if Satoshi-Gekkouga were replaced with Mega Lizardon Y or Mega Jukain.
And for all the complaints – not yours, just in general – that Satoshi-Gekkouga is an asspull, I'd argue that it would be the same if Satoshi were to break over a decade of consistent characterization (since Houen) to bring back Lizardon, or any other old Pokémon of his.
Ash-Greninja is all over the place. It's not an example of good character development because there is no tangible progress, and it's working its way up to having an insane amount of plot armour. I don't like it and I don't think that's going to change somehow. I'm almost looking forward to Ash being reset to default and leaving all his Pokemon except Pikachu behind when he moves to Alola.
Satoshi-Gekkouga being "all over the place" is kinda the point, I think, because it's effectiveness depends on how in sync Satoshi and Gekkouga are. This is representative of the difficulty that Satoshi is facing in learning to master it. I'd argue that you see more visual evidence of their progress than you would in a normal Mega Evolution, since Satoshi-Gekkouga reaches its "perfected" form whenever the two are completely bonded.