I think what makes Shazam magically "just work" is that they are searching for the exact embedding of the recorded version of the song - it doesn't work for a cover version, acapella, etc even if it is very similar. (Everyone would want an app that you can hum into and it will ID the song, but I guess it's a very tough problem.)
Merlin is solving the more general problem so makes sense that it's much more finicky and less accurate.
Merlin is solving the more general problem so makes sense that it's much more finicky and less accurate.