I don't know how to feel about Greninja as of late. On the one hand he's this amazingly speedy character with surprising KO potential, some very unique specials (which happen to be really good) and excellent range on a lot of attacks. But he just struggles so much in the neutral against some characters because his hitboxes are SO precise/bad in general. Like, retreating Fair is a really good poke but that's kind of it, and if you only have one option it's gonna get predictable fast. It just feels so hard to get hits with Greninja sometimes and the top tiers don't seem to have any trouble hitting him.
Maybe the shuriken nerf was a good thing, it sort of exposed Greninja's weaknesses and if it hadn't occurred we would have found ourselves with a character that gets by on one very simple spam tactic. Even though it didn't seem like that was the case before the nerfs, perhaps it was... who knows. It's like how people say Ganon would be stupid if he had his old autocancelling Dair in this game, but he'd only be stupid because of that one tactic, and that's not really healthy. I need to experiment more with Greninja's Dtilt, Dash Attack and jabs in the neutral, all of those options seem like solid pokes that don't put Greninja massively at risk.
I really want to believe Greninja has a lot of hidden high tier potential akin to Peach, but it's clear he's not as technical as her (she has a completely unique, exclusive mechanic which can even be used like a wavedash) nor does he have as many attacks that are obviously good, safe, and easy to hit with. Amsa's performance proves that the character at least has something, but it could be nothing more than people being unfamiliar with the matchup. It's also really, and I mean really disheartening to me that whenever actually good, respected players put out tier lists, Greninja is quite close to the bottom. This happened with Dabuz' and Nietono's tier lists, among others. It's especially worrying to see from Nietono since he used to play Greninja but dropped him. Plus Greninja is supposedly considered "very good" in Japan, that being one of the major reasons he was nerfed so badly.
I think Greninja's potential lies in the fact that his advantageous state is SO strong (I think it rivals heavies in terms of damage output and option coverage) and Hydro Pump is still a no risk, all reward edgeguard that can get kills at stupidly low percents. However once you start fighting good players it seems really hard to get to that advantageous state because his neutral is just so weak. Then you have characters like Sheik and Falcon with excellent neutral and strong advantageous states as well. I dunno, I don't mean to complain, I'm just venting, I hope someone can cheer me up haha.
EDIT: Oh, I did want to add something positive that I've found recently; Greninja's running shield seems to be really good. Like, he transitions into his running animation out of his dashing animation very quickly, which gives him a pretty safe approach option. Every character can use running shield obviously but Greninja's is a cut above most because of his fast dash speed and quick dash startup. If anyone has the frame data on how quickly Greninja can perform running shield that'd be great, since I can't find it anywhere.