There's mmo, mmo rp, and rp and each is unique. the question is, what do you mean by rp? small chat group? a party of adventures traveling the world together and getting fucked by events in the world, or just the mmo, lets all be lewd together type game?
mmo is just lots of lewd things (scarlet blade), which often isn't able to deliever an experience, only eye candy, often these games are not developed enough to be able to make a guild and be able to have a shared experience, and that requires making friends which these games often don't have features to support such interactions, everyone just solos it.
mmo rp, is rping in an mmo world, second life, and while you can do cool things in an rp world, there is the challenge of learning and being accustom to seek nsfw rps with others in a virtually public setting. This isn't helped that second life is a mix of lewd and non lewd playbase, where there are many who will judge you (ie guy rp as girl can be found as a female player with no mic on). while finding a partner is great, then there is the challenge or the question of, what is it to rp?
many rp's (G+ days were a good time for the nsfw rp community, often rping a lewd image with a bit of context, the image helped get attention, and dedicated community helped filter out those who would or wouldn't be interested in your kink) tend to reduce to 3 different rp's. 1 there is the holy grail where you get to experience a story, a world, and that is great. 2 Then there are the dom sub rps which I often find is just people who want to rp but don't want to put in the work so they ask to be dominated and you just end up doing the whole rp one sided as the dom. Lastly 3 there are the few rps where the other wants to be a dom, but this is usually a by product of how most rps don't turn out how they want them too so they try to take more control over the story to guide it to a particular narrative, but either the pacing is to quick and it ruins the experience, it feels like a bad rp or if they do go slow enough it is still too slow for the dom who already knows where they want to be with the story, so its a no win situation. Both have to put in equal efforts and expect the same speed in order to have a good rp, yet I can't find a modern rp community that isn't on some old 2000's website full of 40 to 60 year olds who aren't on the up beat.