Anything can be a writing exercise, it all depends on the restraints you put onto yourself. I suppose it is made far easier by being something many people already have personal experience with, but even if its without any surreal elements I believe slice of life can have complex characters with both static and dynamic personalities. Either way, you're practicing writing what you're writing and I believe a lot of RPers struggle more with side/normal characters, always focusing on main character status dramas and arcs. Being able to rp a character in slice of life could build the same skills to allow you to make a realistic background for a side character in another story or rp.

The true challenge for any supernatural/fantasy RP lies not with the characters though, but in setting. I always overthink any fantasy setting to ridiculous levels, even if its just the normal human world with magic attached, I find myself spending hours considering how the world fits together with the existence of whatever fantasy elements I add. Characters then are based on that setting and by comparison are not very difficult. 

I can't argue the easy part though. I chose it because I wanted to get started on here and didn't want to have to spend said hours upon hours developing a fantasy world for people first, especially before I knew I'd get bites and some solid RPers in there. I just don't feel there's much difference in difficulty for the RPers. Anyone in any Rp can play a challenging character to write, or an easy one, and that's all based on their personal choices and preferences.