I rarely fly at night and would not fly over really dark terrain unless I was close to home anyway.
Flight over cloud whose base is say OVC005 is not good either.
But everything is a risk and whether you get it depends on how often you expose yourself to it. One should always have an escape route and flying at night means you don't have one. Likewise over water without a life raft (lots and lots of people, including flying school sponsored fly-outs, do that).
But with the escape route covered (e.g. carrying a raft) I don't see a problem.
Just my view.