In my experience the coverage is shockingly good. And if you find a case where there is no tldr, you're back where you're started having lost not more than five seconds. The failure case is graceful, so it was easy to put tldr into my workflow.

I wonder if tldr returns a meaningful exit value that you could write an alias/function for something like tldr $term || man $term

Definitely; tldr returns 0 on success, 1 on failure.

    tldr_or_man() { tldr $1 || man $1 }