Personally I find fiction much more challenging than non-fiction, which is why I joined this forum. I didn't need to look for any company or support so long as I was just pottering happily round my comfort zone, but as soon as I ventured outside it, I realised I wanted to talk to other writers.
My previous books did require specialist knowledge (they're based on my PhD and subsequent research in the field of peace and conflict studies), but I'm not finding fiction any less demanding in this way. My novel is requiring a lot of research. To make a traumatic birth scene credible, I had to borrow a relative's midwifery textbooks, read up on birth defects, and quiz my relative about the times she'd assisted at births like the one in my book. Doing the research and building up the knowledge is the easiest part. Far harder is finding confidence, developing a consistent narrative voice, and pushing past the urge to scrap it all and start from the beginning. With narrative non-fiction I already have these skills, but as far as fiction goes, I'm a novice.