If a user tries to join two tables and the joining columns are incompatible the join will fail, even if there would be a valid match if the columns were the same type. Allowing the join even though it will result in 0 matches can be confusing to a user, especially one who is unfamiliar with how things work at the database level.
Instead, a warning or error should appear in the Join window letting the user know that they are about to waste their time, both during the join as well as the hours of frustration afterward trying to figure out why it's not working when it looks like it should.
