I was in a session earlier this week where I was playing recorders, among other things, and there were two people with parallel bore whistles which sounded predictably terrible with the other instruments, me especially. So I fired up ClearTune and demonstrated why. Whistles get progressively flatter through the second octave: you can compensate by blowing harder, and I tried blowing hard enough to get these in tune. Octave D: fine. E: doable but you have to make it pretty loud. F#: you really don't want to be sitting next to someone doing what it takes. By the time I got up to A, I had a whole roomful of people crying uncle.

Even with somewhat better whistles than those, you have a problem: unless the player is confident enough to play VERY loud, they will always be flat. Which puts you in a bind if you're a beginner and don't want to dominate the group.