SongLink supports both URL Scheme (since v2.3) and Universal Link (since v2.0). Currently, you can use both keyword and parse actions.
In short, if you have SongLink installed, you will have a more stable experience with the URL Scheme. On the other hand, Universal Link is better when sharing with others.
SongLink will search directly using the fields passed in by keyword.
SongLink parses the incoming data and then extracts the song information to search. It is commonly used to search for links shared by other platforms, and is also highly fault-tolerant of incoming content.