I suppose i'll give my two cents.

Challenge with original RPs is interest on the whole.  Generally why original RPs tend have a overarching genre that players that invest their own creativity into as opposed to having someone make all the content for the RP.  Also there's the challenge of making sure things stay internally consistent and no one makes something that is totally out of place.  Which happens pretty often, and of which i've been guilty of on many occasions myself.  But the trouble with original RPs is that you're going to have a hard time using the same setting twice, which frustrating since it means you gotta keep building from scratch in hopes of making half a page of IC because you get people who spend so much time making the setting and than lose interest two weeks in. Even if you reboot people might decide to make a whole new society or on the opposite extreme literally copy and paste their other nation without looking at the context of the reboot.   

I prefer making creations specifically tailored to an RP as opposed to trying to force content templates into other RPs which means that every RP I join (with reboots as the exception, I suppose) I have to think up something from scratch and it sucks when you build up for a week your whole society, characters and potential story arcs for it all to never happen because over half the people flake out the second IC starts.

In Fandom RPs or Academy RPs for that matter you just take some other guy's setting that many other people also like and plug in your OCs and walls, IC time in just one day.  This is especially the case with fandoms that are common as opposed to more obscure stuff like a GURPs world or some dude's alt history timeline you want to RP in because many who join a fandom RP probably have a OC in mind from the get go, which makes it all the easier to go from applying to in-character.  

While I will admit there's some creative thinking that goes into trying to make characters/places that would fit in to whatever franchise you're using, in general it's a lot less effort than making everything from scratch simply because of everything that's already there.  You know what a orc is, you know what a balrog can do, you can look up what the Dothrhaki are like culturally or what a light saber or powered by, especially since those who join a fandom RP know all this stuff going in and don't need to do much research beyond what other people are doing.