The only places I can think where you can’t long rest are the mind flayer colony, the nautiloid, and the nightsinger’s astral void. I think everywhere else is good to go for long rest as long as you aren’t in combat.

I thought of another one, Bhaal’s temple you can’t rest in. You have to go almost to the waypoint on the bridge outside it before you can rest again. Iron throne and the Foundry also.