What sounds good is obviously in the ears of the listener, but the use of sound technique is something only a musician well-versed in the technicalities of an instrument can judge. And bad technique can often lead to a shorter career, or at least a shorter time being able to sing well, which has certainly been true of Ozzy. Meanwhile, Bruce sounds outstanding, possibly better than, he did when he started. His vibrato has gotten clearer and steadier and I think he sings the softer parts better, which is very useful given Maiden's current direction. I wonder what this expert would have thought of Bruce's early 90s technique, which sounded pretty bad and seemed to have led to poor performances on the early 90s live albums. Maybe he was screwing up his voice during that stage of his career.
I'm always interested in guitarists' take on guitar players as well. I often find that the ones they think are the best usually don't do much for me. A guy like Ronnie Le Tekro seems to thrill other guitarists, but I think Yngwie Malmsteen and Paul Gilbert give the impression of better playing to the listener.