Some people judge characters with powers like this because there are many people out there who have made characters with powers and were pretty much Mary-Sueish and powerplayers. I see it rarely happen nowadays, but I remember witnessing fights between characters with powers and characters with no powers. Often times the ones with the powers wouldn't RP fairly and start insta-killing the "mortal" characters.
And that's where people form the assumptions of "All characters with powers are powerplayers and Mary-Sues!" Is it true? No!
I've seen many characters that had a power or special ability, but they had flaws too. I myself have some characters that would classify as "immortals" because they are based on immortal beasts of fairy tales and make-believe. One such character of mine is Amalthea the unicorn fox. No she is not the average fox. She is a white fox with a unicorn horn and unicorn powers. She is somewhat of a lesser forest deity. Even though she has magical abilities, she has plenty of flaws to follow. She can live forever if she wanted to, since she can never grow old... but that doesn't mean she can't die. She can be killed. Unicorns can be killed! We've seen that happen in movies like The Last Unicorn, Legend, and Harry Potter. Plus, Amalthea's magic is very limited. Using magic to heal the injuries of someone can take up her energy and weaken her.
As for my manticore character.... she doesn't really have any magical powers like some dragons, griffins, and other manticores have. She's just a big beast that relies on her strength as a power. But she has a few flaws.
1.) She's an old manticore, and not as strong as she used to be.
2.) She's a slow flyer.
3.) She get's tired real easily, especially when it comes to combat.
4.) She acts based on primal instincts rather than reason.
These two characters have been labeled as the Mary-Sue powerplaying type, but that's because they weren't given a chance. Sometimes people who are too quick to judge a character based on their species and supernatural abilities don't allow the creator to explain themselves and really don't give them a chance. They be all like "Oh NO! He/she has powers so he/she is automatically a powerplayer! No way I'm going to RP with them!"
You know what they say, "Don't judge a book by it's cover". That saying can apply to characters here too.