For example, a file named foo:bar does not appear in the search results when grep is used as the backend.

Howm runs the following command internally and parses its output (howm-grep-parse-line). A colon in the filename obviously confuses parsing here.

echo 'foobar' | \grep -F -i -Hnr -f - ~/BAZ/QUX

I don't believe there's a perfect solution for the default grep command. If you really can't stand it, probably the only way would be to use a non-standard version of grep that supports JSON or something similar. Personally, I don't think it's worth worrying about that much, at the cost of losing its out-of-the-box usability.

Is "won't fix" reasonable?