Some people just have it tattooed on their back.
Everyone gets on over-powered characters, but what about the under-powered ones? I don’t mean this in a strict sense; you can leave the power level chat for another day. I mean in terms of the character’s victories or defeats over the course of a story.
It is important for a character to strike balance when it comes to triumphs and failures. The two aspects play off each other, and lend substance to each other. Losses have impact when the hero has something to lose. Victories have impact when you overcome a loss. If a story has too much of one thing it can have a negative effect on the reader.
This balance can be disrupted on a character by character basis. If you have a character where nothing but good things happens to them the reader may start to get bored or irritated with the character. They may pine to have the focus shift to other characters, or worse they may put down the book.
The opposite, someone who fate just keeps kicking when they’re down, may irritate the reader as well. While both of these can be offset by how the character reacts to the circumstances, it can only do so much.
This is why weak, seemingly insignificant characters in stories often get big moments. Even a character dismissed as comic relief may make a contribution leading to victory. Think on the story of Rudolph the Red-Nosed Reindeer. His point of ridicule ends up saving Christmas.
When a character strikes me as infallible and invulnerable, my writers instinct braces me for a fall from grace. It’s just one of those things I notice in great stories. Also characters bogged down by mud I expect great things from. I think people gravitate towards balance by nature. Though, It’s always good to throw a curve-ball every once in a while.
Sometimes you need to disappoint your readers, make them expect one thing and dash their hopes on the rocks. You don’t do it just to be a jerk you do it to leave them with substance and deepen the immersion of the world. A happy ending is nice, but what is it worth if it comes without cost? Tragedy makes joy so much sweeter when they work in tandem with one another.
Memories of a Dimanagul 
Note to self: add “save Christmas” to the bucket list.
Not to go off on a tangent, but if I had to guess, I’d say it’s easier to give overpowered characters more trouble than underpowered ones. I can imagine the mindset: “This guy is too strong! He never loses! How unrealistic!” Maybe the bottom-tier characters can slip past because they’re (somewhat) easier to identify with. “He’s down on his luck! He never gets his way! His life is a joke! I sympathize and identify with that!” Well, maybe not in those words, but still. I’d like to think it’s a possibility.
Of course, I’m not out to try and invalidate what you’ve said here. You’ve got it right; strong or no, characters need to have their ups and downs as they go on their merry way. It’s all about those high-low mixups — rely on the same moves, and you’re bound to get bodied.
…I think I need to give Street Fighter a rest for a good long while. Then again, the new Half Pipe theme won’t get out of my head. EVER.