Depends on the Ren fair, trans buddies of mine are frequent visitors, performers and enjoyers of them for decades and rarely had they problems (read on the level of every day stuff, with drunk people a bit more; rarely less). But the ones where they had problems they often had problems continuously.

This is speaking about German city states and some Ren Fares outside, but also for Danish ones.

For negative experiences: In Germany you have a couple of nationalist rigth wing folk people on some Ren Fairs, who try to connect with “Pagan” “Eso” aesthetics to the environment. This might not be the case so much in other places.