On smooth streets I like a roadbike at night. On pothole-and-other-hazard strewn routes that I can see better in daytime to avoid when riding my roadbike, at night I use a heavy-duty bike so I don't worry about hitting them. (I have great lights, a dual set-up that gives 1600 lumens on full power, and as wide a coverage as I want to set, but even so...)

Also small animals crossing. Heavy bike beats light bike here. Not that I have ever hit a possum, skunk, cat or rabbit, or run into a pack of night-roaming dogs, but if I did, I'd much rather not damage my bike and body parts. Deer? You lose, no matter what you are riding. Also my MTB has lots of reflective tape and monkeylectric lights, which I don't want to junk up my roadbike with.