I've kind of changed my mind on the underhand serve because of a recent experience I had. I used to think underhand serve was perfectly fine; I no longer think so.
A few weeks ago I was playing a doubles match with some friends. I was returning. The server stepped up to the line, I was ready to return. He hit an underhand serve - but I'm fast, i ran forward to get it! ....and it turned out he hadn't hit a serve at all. He'd tapped the ball to his partner at the net, since he had all three of the balls and wanted his partner to hold the third one. His net guy picked up the ball and the server got ready to start his service motion.
Oops.
Do I get to call that a fault and make him take a second serve?
What about the other way - if the player underhand serves, and I don't realize he's starting the point and think he's clearing a ball out of the way, so I don't hit it - what then?
So that's kind of crystallized in my mind why I don't like underhand serves. Because it "works" not by disguise about where the ball is going and how it's hit, but by trying to "disguise" that the server is starting the point.
If you take a look at any pro match, there's going to be PLENTY of times when the server gets a few balls from the ballkids, picks out one, and then taps another ball with their racquet to pass it to a ballkid. Does the umpire call "fault" on any of those? No, of course not. Because they're obviously not attempts to serve the ball - you can tell whether someone is actually serving the ball, instead of just tapping it somewhere, because they have a clearly visible service motion, with an attempted ball strike at the end of it. But if surprise underhand serves are ok... what exactly makes it clear when the server is tapping the ball without an intent to serve, and when the server is actually trying to serve? "If it goes in, I was serving, and if it doesn't go anywhere near the service box, I wasn't?"
(Obviously, that's not the case when someone clearly shows that they're starting a point with an underhand serve - if they have an identifiable and visible "underhand serve" motion, so it's clear to the opponent that they're serving. That's different and IMO still perfectly fine.)