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.