A lot of fanfics treat Ranma like a Martial Arts god with unique, amazingly superhuman powers. (Heck, I've done it, before rereading the manga and actually thinking about things.)
This is due to a lot of reasons. The usual Mary Sue Author Insert Power Creep that infests literally every fandom ever is part of it. But there's other problems. Even writers that don't try and wank Ranma make a lot of mistakes about the setting and the "power levels" of the characters.
The Supernatrual in the Ranma 1/2 universe is fairly common
This, is, perhaps, the biggest thing. So many fanfic writers take the Ranma 1/2 cast, and transplant them into a world that's more realistic, more like our own. They do this with their abilities intact, and suddenly they're basically superheroes thrust into a normal world.
In the Ranma 1/2 universe magic is not something that anybody is particularly shocked about. Jusenkyo curses are treated as a mildly embarrassing problem. The Tendo's reactions to them going there is "Why did you go to a dangerous place" like cursed training grounds are just a thing they're aware of and know to stay away from.
Power Armor that give you super strength and speed (that then explode) are sold in mail order catalogues. Cursed paintings are shown off at festivals, you can buy magic fishing rods that make people fall in love with you for cheap in gift shops.
The Supernatural isn't anything special in Ranma 1/2. And since the supernatrual isn't anything special, that means that the supernatural feats the Ranma 1/2 cast performs aren't that special.
The Martial Artists in Ranma 1/2 aren't superhuman.
"But wait!" You may say, "They punch through walls, jump through ceilings, shoot out energy blasts" And yes, that's all true. But that doesn't make them superhuman.
Do you think Olympic athletes are superhuman? No, they're not. They're still human. They're athletes. Through talent and practice they can perform feats better then your average person. This is also the cast for the bulk of the Ranma 1/2 cast. Through talent and practice punch harder and move faster then other people. But this is just because they worked for it. They're impressive, yes, but everybody in the Ranma 1/2 universe seems to have durability and endurance beyond real world humans, even taking into account Slapstick Comedy Filter.
We do meet some characters that explictly are superhuman in Ranma 1/2. The Musk and Saffron.
In both of these cases they're shown to be absolutely stronger then anything the regular cast can pull off. Mint is faster then anybody can match, Lime has more strength then Ryoga or anybody else. And Herb? He was able to shoot insane amounts of Ki blasts like nothing. His entire fighting style was basically Go Full DBZ, something nobody else is able to even do.
The Shi Shi Hokoudan is basically a pale imitation of that strength. It's also not a fighting technique, but used by miners and is explicitly said to be really easy to learn. It's not that good.
Same with Saffron. He's explictly a magical being and it shows. Hurling insane power and fire everywhere. Ranma only survived thanks to having a magic item that just happened to counter him. Which leads to the final point
Ranma is not a Martial Arts God.
Commonly Ranma is treated like the Ubermench Martial Arts genius. He can learn anything and do anything.
Now, frankly, this is a load of bull. First off, a good half of Ranma's "innovations" which people point to to say he's a super genius are thanks to other people.
Ranma was perfectly content to continue to slam his head against the figurative brick wall to learn the Chestnuts Roasting over An Open Fire speed technique until Akane dragged him to a festival to take a break and he saw the fish tank. Same thing happened with the Moko Takashiba. Ranma was forcing himself to be miserable in an attempt to learn the technique. It wasn't until Genma pointed out that he could adapt it to a different emotion that he figured it out. (And then the Moko Takashiba turned out to be useless. ¯\_(ツ)_/¯ )
This isn't to say Ranma isn't good. He is. He's obviously a talented martial artist, one of the best of his age group. But he's not going to be fighting at Cologne or Happosai's level any time soon.
The fact is, that Ranma's victors over opponents stronger and more skilled than him makes him a more interesting character then a Martial Arts God who instantly figures out how use Every Technique Ever.
So yeah, remember this. The Ranma 1/2 cast aren't super unique or superhuman, magic is a regular thing, and Ranma is good but not That Good when writing fanfics.