The obvious approach is that you look for "people who like this, like that". This is easy to measure and understand. And when you're faced with a person who liked this, it is usually the right answer.

The problem is that you don't know WHY people who like this, like that. Often it is simply, "Heard both at an impressionable age." What you want it to be is, "On the musical criteria that I can recognize, this music is similar." But that's a complex piece of logic, and it is not clear how to do so.

Which makes the tools really good for rediscovering all of the stuff from your childhood, and really bad for discovering new music that is similar to stuff you like.