Greninja isn't nearly as safe as Pikachu or Sheik. If they miss their chance for a combo, no problem, they barely put themselves at risk to do it, they can do it again anyway. Greninja at least has stronger KO moves than them, but he needs those because he's not even remotely in the same league as far as frame data. The combos don't all work on every character, even if he needs them to KO, is what I'm saying and something to keep in mind.
Greninja is evasive because he HAS to be evasive, not just because he can be. He doesn't have the frame data or hitboxes to be anything else. He can't afford to not be evasive. And you'll notice games where Greninja is involved often go below 2 minutes left. As for inefficient, I guess it would be more like that he doesn't keep up the pressure the way you might expect. He's a fast character who's very reliant on footsies and spacing, which is not what you expect from a "fast" character. Look at the Some video against a Captain Falcon (
https://www.youtube.com/watch?v=pcp2kSTN8Lg). He's frequently standing still, throwing a shuriken, or running away. When he does get a hit, it's a single hit that doesn't lead to any meaningful pressure or followups. Obviously this isn't unique to only Greninja, but as a fast character who doesn't have the traits you would expect of a fast character, it's noticeable. He's fast and trick people into thinking he's dangerous, but if they understand when he is and isn't dangerous, such as not airdodging after a down throw because Greninja doesn't even have a followup, then things can drag on. Of course he at least has a KO throw, so if you miss or never have the window for a KO setup, it's not too demanding to get them to 150%+ and up throw. Might be too hard on the character to describe it as "inefficient."
