I'm fine with fandom RP, but I largely prefer if there's an OC aspect in it.
My favorite type of fandom RP is Concept-based, so the settings may not be somewhere/sometime within the canon universe, but the rules and the basic trade of the universe will be based on that canon. I remember that I made quite an in-depth OC Persona 4-based fandom RP, where the story will have little to no connection to the original canon, and the canon character is only mentioned in passing, but the whole concept of the RP is still similar to the original P4, where there's a group of people with personas, some without, and they would go into the TV world and solve murder mysteries. This kind of fandom RP is right in my book, because there are several fandom world that I'd love to be playing in, but more often than not, I just like the technology/magic/power/supernatural stuff that's in the fandom, not particularly the timeline or the settings.
It's also rather easier to RP an action based RP with an established concept that both you and your partner understood, so you don't have to think too much/explain too much about the rules and physics of the universe, reducing the need to argue about a seemingly OP moves. This however mostly applies to fandom that has their own rules and physics, so stuff like Star Wars, Persona, Marvel/DC, Digimon, RWBY, and those kind of stuff. I'd argue fandom like Rurouni Kenshin, Hajime no Ippo, Yakuza (the game), or Black Lagoon doesn't quite offer the same amount of rules and physics into the story, since they are rather based on realism than anime physics, and rather than RPing in those fandoms, I'd rather expand the scope of the RP, so rather than Rurouni Kenshin RP fandom, why not just do a samurai-based RP instead? Instead of Ippo, why not just about a boxing based one? etc.
The absolute most I can do with fandom RP is with location and timeline, not ideal, but it helps with the worldbuilding stuff, so we can jump into the story. Works rather well if you have a partner that's not too keen to help with the worldbuilding aspect. Sometimes, location and timeline is rather tied with the concept as well, so in that sense, I'm fine with that kind of stuff (since you can't have Star Wars RP in the past, or MMO-Isekai type of RP without the modern-day + virtual world settings).
However I am absolutely against RPing solely as a canon character, for either me or my partner. Just like how others had shown, canon character has a predetermined characteristic and personality, and add to that, the one RPing them and the one who wants their partner RPing them would also have their own preconception of how the character will act and talk. I might not like how my favorite character is portrayed, and my partner might not like how their fave is portrayed, which would just lead to an argument over argument.
In my humble opinion, RPing is where imagination comes to life, and with fandom RP, I just feel like there's just a lot of things that inhibits that imagination. I'd rank concept > settings > characters in terms of that restriction, with characters as the most restrictive type of fandom RP. I type directly from what I envisioned the scene would be like, and it's a hassle to have to envision the scene, THEN envision what would your character, a canon character, would react to that scene canonically.
Not only that, this is talking about when you already got a partner to begin with. With character based fandom RP, the net you cast is way smaller compared to what you'd cast when doing an OC RP. Not only that you have to find that certain someone that loves that fandom to begin with, which gets smaller the less mainstream your fandom is, you have to find a specific someone within those net that will do the pairing that you'd like, which gets even smaller the less popular the pairing is, AND THEN you also need that person to fulfill your requirement as an RP partner, kinks, writing proficiency, writing pace, that kind of stuff.
So yes, 95/100 times I'd pick OC RP than fandom RP. I don't hate it, but I just love the freedom OC RP gave.