I didn't include Samuro because doesn't have access to permament Stealth.
However Samuro isn't really good Hero. Yes, he has very high single target DPS, but mainly against single target and your Images are attacking same target. Other than damage, which feels very clunky and unreliable, he brings no utility to fights. He is also pretty strong at 1v1, but he struggles at forcing those fights because most Heroes can just disengage from him.
What he does good is being a semi-global Hero with Illusion Master, swapping between lanes on demand; and taking mercenary camps. Zeratul brings similar global presence because enemy team can't identify his rotations and after level 13 with Mending Strikes he can take Giants just fine, but does way more in team fights.
Yeah, you are right. It is about burst damage. However for some reason, people have trouble accepting Rogues sneaking and stabbing them into back, but when they get blown up by Pyroblast from Mage standing a mile away they are fine with it. To me it seems like some irrational psychological issue more than anything.
About the Mount speed, in HotS there are two types of stealth. First you have Cloak mechanic of Zeratul and Nova who are have it permanent and available while mounted, but are unable to active during combat and the second is Samuro's Windwalk and Valeera's Stealth who can use it during combat, but can't mount. Completely fine and fair for me.
Sure, every Hero can punish a bad play, but not to the same extent. Stealth Heroes are better at it because they can move undetected and scout situation before committing to the kill. There are way better duelists than most Stealth Heroes, but if they are roaming and encounter a full enemy team they are free kill, meanwhile a Stealth Hero can follow a prey and strike when situation presents itself.
I didn't say they single out opponent, it is usually done by Warriors and mobile Assassins.
Stealth Heroes are mostly being played as semi-globals because they scout, harrass enemy during rotations, join for ganks or can even safely soak side lanes. Since you asked about Genji, he can do those things too. However in fight he serves slightly different role. Zeratul usually waits before committing hard to set up combo with Void Prison or burst target down. Genji on the other hand can harrass a single target for long period of time or move back and be played as 'frontline' Valla, because his defenses allow him to do so, while waiting for resets.
What I am trying to say, Zeratul needs to be more careful picking his fight because he doesn't have Deflect, huge gap creating ability and access to 3 charges of AA negation.
I think Stealth should be tool to scout and set up ganks or combos. So far only Zeratul does that while Valeera has the potential, but her melee range on abilities and lack of sturdiness prevents her from fulfilling it.
No idea about Nova though. I think she needs a bigger rework because she is easy to punish once revealed and her current kit doesn't have anything to set up combos. Or just scrap the Cloak mechanic and give her stealth she can use in middle of combat.