I usually sing the "Unquiet Grave" when in doubt and the audience are a little serious. Having said that I am often on the verge of humour but never quite there, and plenty of pathos.
I am known as a joker so maybe they wait for the punchline that doesn't come and applaud out of relief.
I can't say it never fails but almost never. Maybe I read the audience.
A few times I sang it with one of those voice operated wagging flowers, and it got more laughs still in the verse about dying flower.

having said that there are plenty of times with some songs when the silence was so oppressive I wished I had chosen "safe".