If the story acknowledges the existence and pivots on Spider-Man existing in a milieu where such options are available and accessible, i.e Spider-Man's story and situation is based in a context where there are multiple superheroes, teams, and magic and super-science are real. Then it's a problem. There is a certain amount of willing suspension of disbelief where a reader can get behind. For instance the Master Planner arc has Aunt May irradiated and needing a cure. Now at that time, Spider-Man did have a shared universe and had a few encounters with the Fantastic Four and logically one can make a case that the story could have ended had Peter gone to Reed Richards first. But the story and context makes it clear that this story is about Spider-Man solving this and doing it on his own. The situation and context is completely grounded in Peter's corner every step of the way, and indeed it is among the most purely standalone stories, where the non-initiated can believe that Peter's the only superhero in the city. The Marvel Universe was originally a gentleman's agreement whereby each story acknowledged the existence of other heroes but it was understood by editors/writers and readers that every story is standalone and can be perfectly understood as the only hero in that situation. And the shared universe was largely for the benefit and aid of characters like Iron Man who could not entirely lead a story/title by themselves.
In the case of OMD, Aunt May gets shot but the bullet isn't immediately fatal and she gets injured and taken to the hospital and is in medical care for a very long time, long enough that Peter has the Back in Black Arc, you have tie-ins in different issues and stories. In fact, JMS actually cited this as one of his problems with that story. His original plan was that Aunt May would get shot and that would lead directly to OMD, with Back in Black and other stuff happening before that. But for some weird reason, Quesada insisted that May get shot, the other stories happen and then OMD comes at the very end. This by itself conveys a situation that Aunt May's bullet wound could have been recovered and fixed and it also dials down the urgency and desperation that is needed for the Mephisto deal to be believable. Then you have a story that has Peter going and visiting everyone in the Marvel Universe to save this which is supposed to drive the desperation, Peter knocked on the door of everyone in Marvel and all of them told him it couldn't be done and whatnot.
So you have a failure of execution on every level. The situation is not desperate enough, the lack of desperation means that the likes of Reed Richards, and others should be able to fix this. Not enough is done to suggest why this is beyond him. Like if May was shot in the neck and bleeding all around and Peter desperately webs all over face to stanch the wound and he has to keep doing it every so often before the web-fluid dries up, and he's on the run and can't make the web-fluid fast enough.
That only becomes a problem if Reed Richards is shown curing cancer or AIDS. Bullet wounds or serious trauma injuries are survivable and recoverable. People do survive shootouts, battles, terrorist attacks and accidents all the time. So it's on the believable level.There is also the problem of what it means for stories in which someone is sick if Reed Richards has much more advanced medical technology than the modern world, and is able to save anyone who is sick.
Aunt May didn't die of sickness, she got shot. Reed Richards is not being asked to cure sickness, he's being asked to operate and fix a bullet wound.
Peter defeated Juggernaut and Firelord the same way and many others. Doom refused to bargain and reduce himself to Mephisto's level is what matters and counts.And Doom didn't defeat Mephisto by being moral. He won by being crafty.