Since most of the time, those disagreeing with such characters, don't really give any proper argumentation and just go "no, that doesn't fly" I choose to give a full argumentation of my personal opinion in regards to such.
While it's horrid when things get as messy as harassment level(I am also guilty of such on a few occasions) it's customary that whenever a small-mid-large group of people disagree with a certain form of rp they will ignore and or express their displease.
"Note: Using demons, curse seals of heaven, sharingan, etc that you do not have in-game, will displease many people in the zones. The best suggestion: use what you think is fair for our level."
This is the most we get in regards to rp rules, an advice of sorts, breaking it won't lead to any punishment of sorts on site staff level, and as such, SL is an rp community that rules itself. People will ignore and/or bash(sometimes crossing the line - again, guilty and sorry) whatever they deem as "wrong" in means of rp. And example of the community partially if not entirely silencing a belief would be claiming the in game kg in rp w/o having them or claiming a level far beyond your reset count.
The same applies when someone creates an overly modish jutsu of sorts. Now quite a few disapprove of custom demons and the alike, I will now elaborate my arguments in regards to my disapproval and possibly the arguments in the head of most that do disregard such characters:
It's not in Naruto, yes there are a bunch of fannon things on SL so that on it's own seems weird in spite of being the most commonly used argument. We have a whole bunch of custom techniques, summons and the alike that do not exist in Naruto, the thing would that we do have what to relate them to such as other summons and other technique, whereas for demons, we just have the bijuu which in my view should clearly stay just that.
A second argument would be that it's at least as bad as claiming kg you didn't reset for if not worse. In most case people risk their lives, facing overly dangerous opponents in order to obtain a bijuu while regardless of weather in a lesser, similar, or superior form, just coming up with a random demon that is inside or that you are is simply a cheap way to claim bijuu/jinchuriki like status w/o earning it and w/o the tiresome responsibilities of a jinchuriki. Basically, just by putting it in your bio.
I also have the issue that most of the non-shinobi characters I know fail to rp properly w/o modding.
And now for my strongest argument:
Regardless of what we are, it's only fair that we play by the same rules. If a regular player(shinobi) would be deemed as modish for a certain claim or technique of sorts, the overly common "I can cause I'm a (insert random super natural creature)" shouldn't fly either. My dislike for such characters is greatly augmented by that excuse.
I don't intend to attack anyone with this, I am simply using the first example that came to mind: Kaizer/Nay's teleportation - Were one to come to a technique similar to hirashin or Tobi's teleportation they would be greatly frowned upon for ripping of the techniques, one of them being "claimed" and the other, arguably Uchiha Master+ only. While the teleportation I am addressing is not only similar to the two but superior, surpassing the barrier set on time space ninjutsu, in the sense that it takes the good parts of the two and kicks out the drawbacks. In means of vanishing, it is as fast as hirashin but lacks the marking prerequisite while the draw back kick out on the other time space ninjutsu is that it eliminates the vulnerability part in the sense that they don't take few seconds to phase out, seconds in which Tobi is vulnerable.
To sum up the above, I dislike how being a super natural being of sorts gets them the idea that they can use it as an excuse to claim what for the regular SL player, and in my view, the proper one, would be highly moddish.
Some have the the idea that resenting any form of rp means harm for the rp community in general. In my opinion, opting for a form of rp known to be a sore eyes for many is at least just as bad.
Again! The reason I've made this for is to make my full opinion on the matter a public, I am not looking for any arguments, I've already stated mine and continuing any further would just mean repeating them over and over again. I will by all means NOT respond to ANY form of attack to this opinion or my person, and possibly not even bother to read such.