A night circling approach where you may be able to see the runway on downwind or base but where you cannot judge the height between you and the terrain in your projected flight path, is relatively safe until the moment you elect to descend below the published circling MDA for whatever reason. From that moment on you are entirely responsible for your own obstacle clearance. The lawyers of the passengers are waiting in the wings.
If you cannot absolutely guarantee safe terrain clearance, but nevertheless choose to commence descent below the published MDA on downwind or base in order to ensure a stable approach, then may Lady Luck be with you.
Far safer to hold MDA until guaranteed obstacle clearance within the landing approach splay. If this also means your final approach is unstable in terms of speed, rate of descent, power setting etc, then this again becomes your problem. Either go-around or don't attempt a circling approach under these conditions in the first place.