Then you know he has had lots of off nights those years. Generously, Halford was a very inconsistent live singer, and even on good nights would have peaks and valleys from song to song (for example, finding exemplary versions of Victim of Changes is quite challenging). Pre-79 he seems to have been more consistent, though even in ‘78 he has less-than-great shows, and I can find you plenty of examples of him sounding bad live as early as 1979.
In terms of consistency, stamina and control, I would say that Song So-hee is up there with the best. Indistinguishable from her albums and singles. I guess a decade's worth of Gugak training helps Just imagine what another decade can do, once her voice fully matures!
Compare this pop duet performance a couple weeks ago to two totally different traditional pieces from last year: