It's an interesting question, and I'm not sure I'm the best person to answer it, but I'll give it a go. For me I find it to be reasonably easy to write OCs. The difficulties I have are more to do with my familiarity of the 'verse I'm writing in. I can write Star Wars really easily, and I could tackle Mass Effect fairly confidently too. I always try and go for the bigger picture and pick out bits of the 'verse that aren't well known, so fitting into canon is less of an issue. Writing an OC in the Discworld 'verse would be another matter completely as it's such a well established 'verse with maps and things. There's no real freedom to explore so I'd find it confining creatively and I'd really struggle there. So if you imagine the 'verse to be like a sandbox, what I'm really trying to say is that the bigger the sandbox the more fun I can have with it and the easier it is to have that fun.