Defaulting to the only result in your library is perfectly reasonable, but it the case of a collision, what does it do?
My guess is that it runs through in a loop (I haven't looked at the code) and simply goes with the first result that it finds, but that doesn't cover all possible use cases.
Unlike the -e solution, it wouldn't make the command significantly more verbose than the default option.
You could also print a list of commands for the specific options (or allow for index based selection) in cases where there were collisions.
kew dir <album name> (sometimes it's necessary to specify it's a directory you want)
kew song <song> (or a song)
kew list <playlist> (or a playlist)
The directory can be an artist or an album, so there's still ambiguity there. But kew cannot differentiate between the two. It matches against files or against directories.