In the standards world, it is quite common for someone to play the roles of subject matter expert, technical writer, or facilitator. In fact, some of the best SME's I know are also tech writers or facilitators or all of the above. The best are always very clear about what hat they are wearing and when. When I write as an SME, my writing style varies from when I'm acting as tech writer.
It's important to understand what role you are playing, and when you are playing it, especially in engagements where you have to wear multiple hats.
When I facilitate, I try very hard not to get into the SME role. You can shut down discussion in that mode of discussion if you aren't careful, especially when the group you are facilitating isn't as up on the topic as you are. Sometimes it is unavoidable, and in those cases, I find it helpful to move into a different space, both mentally and physically.
When I read what I've written as an SME with my technical writer/editor's hat on, I can be pretty harsh on myself, but only if I can get the right distance. Again, I have to change mindsets to do that, and not just physical distance, but time is also an essential part of being able to do that. After I've finished writing something with my SME hat on, I have let it rest a day or three before reading it again with a reviewer's/implementer's mindset. And I review in a place different from where I write.
Ideally, I like to work with a team, so that I'm not playing (or even trying to play) multiple roles at the same time. And, so that I can play the role I'm best suited to, and they can play theirs. And when I get to pick that team, look out.