Meh, in theory OCs are okay. Afterall, a well written OC is really just a character. The warning comes with the implementation since OCs can normally be born from SI. Also, OCs generally have a higher trust cliff for readers to scale since they don't know them and can't trust they won't have negative writing traits (i.e. Mary Sues, authors pets, etc.)
Although, I personally dislike trying to put in D&D characters that have to strictly follow D&D's rules since they don't always translate well. (I have this image of Kieran setting up D&D figures and rolling dice to write his story). The problem, as RaM encountered, is simply the fact that a character from D&D has a strict system that quite frankly is narratively expensive to explain and/or keep track of.
Of course, all our characters will kind of be awkward "fish out of water" characters given most of the time they'll be jumping into foreign universes, it just depends on how well you write the characters. Personally, I'd suggest if you wanted to have an OC to have/bring a Fate or Tsuki character to be the main character your OC being a support character you could build on through the story.
As an aside, I just went through Brandon Sanderson's classes on Youtube. Interesting stuff if you guys wanted something to listen to on writing.