Say what you want, but the fact still remains that Sheik is not as easy to use as you claim her to be. This also applies to Greninja, since Water Shuriken isn't exactly the most efficient projectile out there, and most of its approaches are not safe either.
You haven't exactly proven either of these things though.
Like I said, while Sheik mainly focuses on getting up close and rushing down opponents with her combos, she is more than built for that. Her moves have barely any lag and the little launch power on many of her moves means she can follow-up ad infinitum. She can then also choose to zone people with needles and burst grenades, and then can safely jump back into close combat with bouncing fish or just her sheer speed and safe attacks, like F-air.
I could go on, but I don't even play Sheik too much and already I can come up with a lot of very easy, but effective strategies. Mastering Sheik is like a rhythm game - you just need to know when her combos hit and master that timing. That isn't very hard to do. If I wanted to, I could become a competent Sheik player with a few hours of practice, if not less. Sheik also focuses on rushdown and camping, which are two practices that Smash players are more than familiar with, so it's not like I wouldn't have a basis going in.
Greninja is kinda a similar deal, except he's more bait-and punish. That's still something Smash players are well acquainted with, though (
). Furthermore, his Water Shuriken is more than efficient. It's great as an aerial projectile and to threaten space while edge guarding, and on the ground, using uncharged ones while performing backward short hops works wonders for keeping the opponent at bay.
When talking about "how technical/hard to use" a character is, I'm not looking at how hard some people personally feel they are to use, or even how good the character is competitively. I'm looking at:
-Skill Floor for beginners (much of this is defined by how safe they are overall when it comes to throwing out attacks, or just how easy the attacks are to understand - how careless and button mashy you can afford to be)
-Amount of unique advanced techniques they have/reliance on general ATs
-Familiarity
-Whether they are reliant on tried and true formulas or require the player to get creative
Rosalina easily has one of the highest skill floors of all the characters in the game, has probably got the most unique advanced techniques of all characters in the game (Sonic and Pac-Man are really the only characters that might rival her there), she is easily the most unique and fresh fighter in the game (Ryu and Shulk may have technicalities and fresh gimmicks for Smash, but Ryu at his core is still a straight forward brawler character with a focus on rushdown and combos, and Shulk is still a straight forward swordfighter with a focus on spacing. Also, Ryu feels very similar to his Street Fighter II incarnation, a lot of the knowledge from that game actually carries over well into Smash, and there is a lot of overlap between Street Fighter fans and Smash fans), and Rosalina doesn't have much in the way of bread and butter combos and tactics, something Sheik has in spades. Rosalina requires creativity and intuition. And requires the player genuinely think about what they're doing on a case by case basis.
There is a reason why Rosalina is barely used in tournaments compared to characters that are competitively not as good by comparison, and despite her being the ONE truly valid counter to the other top tier in the game. It's because people find her very hard to use, they can't just get good with her overnight and look up a few bread and butter, 0-death combos online or anything like that.
