Relatability is always under the surface, kids like Spidey because many of them are nerds, bullied in school, had a crush on an seemingly unattainable beautiful girl, money troubles. They relate to Batman due to his fighting style, detective skills, cool gadgets that they want to have, they like Iron Man because he is the tech wizard and genius we all dream to be, they like Deadpool because of his jokster attitude
Meanwhile, Superman, God with deus ex machina like powers (laser sights, ice, X-ray vision) I mean just look at this
https://en.wikipedia.org/wiki/List_of_superhuman_features_and_abilities_in_fiction
He is way way way more powerful than any humans can ever dream of, and his only weakness is a rock!
What makes characters relatable are their weaknesses, Spidey's money troubles, Batman's orphan past, Iron Man being a mortal human, Wolverine's past of losing everyone he loves and the curse to live forever. Superman has no weaknesses
There must be a reason that no Superman movie seems to be working for the past 25 odd years, we don't see Superman games, cartoons, animated series that often, even his comics don't sell that well anymore
You've picked bizarre examples here.
Reliability isn't really about having weaknesses, it's not about saying "Oh, he's just like me and doing what I'd do". It's about creating an interesting and consistent character-- It's about understanding why they'd do what they do, even if it's not what you'd do.
Re: Weaknesses and deus ex machina... That's all superheroes. That's practically all fictional characters. First of all, Kryptonite is far from the only thing that can beat Superman. Many, many supervillains (and heroes) have gone toe-to-toe with him over the decades and come out on top, or nearly so. Whether using weapons, traps, or their fists. It happens... exactly the same as every other hero. Hell, he was once
beaten to death. "But he came back to life!" you say? Well, so have many, maaanny other superheroes. Also, how is Wolverine's "I can heal from anything in seconds and practically live forever" somehow MORE relatable? Or Iron Man's intellect? Batman's brain and skills and background? Any of it?
And before you or anyone else says "But, even then, you always know Superman's going to win in the end!"... Are you really sitting there with stories about Batman, Spidey, Iron man, Wolverine, James Bond or Sherlock Holmes, Captain Kirk or Luke Skywalker, or John McClane (the ultimate every man, for many) and thinking "Oh wow, I think he's actually going to lose and die, and the Earth's going to blow up, and that'll be the end of the series!"
Superman has the DP Clark Kent persona-- That's the "relatable" in the sense that you and others espouse is so important. That's the side that has to behave like an invisible coward (compared to Supes), is lovesick for Lois and pines for her, is occasionally seen as an easy target by Lombard and others, needs to do his job or get chewed out by his boss etc. And even as the ultra-powerful Silver Age or All-Star Supes, THAT side of him grapples with complex emotional issues, to-- How to best go about saving the world, wondering about Krypton and the parents he never knew, dealing the problems he has so far failed with (restoring Kandor, rehabilitating Luthor) etc. Yet... even through all that... Even though he may question how to go about certain things sometimes... He absolutely believes in the fundamental good of himself and most everyone else. Honestly, more than any other (mistakenly) perceived lack of physical weaknesses... I think
that's what a lot of the characters critic's have a problem with-- That morally, he's
kinder and better than we'll ever be (even though he does get angry and sad on occasion). Personally, that's what he always should be. That's just on of the core reasons he's always been and always will be such a brilliant character... He's someone to
aspire to.
All the most successful, acclaimed versions of the character over his near 80 year history of being one of the biggest fictional icons in the world ("Oh, people just don't like him" my ass) are the ones that EMBRACE all that with open arms-- From George Reeves to Chris Reeve, Silver and Bronze Age Supes, All-Star and Secret Identity, Kingdom Come and Red Son (stories about him reaffirming that side of himself)... They're all kind to a fault, all unapologetically good either all the way through or by the end.
Those that aren't on board with that, who don't get it... Well, you end up with what we're getting right damn now-- a disappointment.