Can a former singer with a flawed technique become a good voice teacher? Instinct tells me that makes no sense.
Until recently, I had never heard the name of Chilean tenor Mario Antonio Diaz Varas, but it turns out he's a teacher at Mozarteum University, and quite a prestigious one at that, it seems. At least quite sought after.
If one looks up his name on youtube, only a few videos come up, some rather horrible samples of his art at singing (uploaded by himself), and a three-hour and a half masterclass.
Extremely ingolata and pushing, puffing his throat like a toad. The whole pack, artificial darkening, guttural, all vowels turned to "O", wobble... Not surprisingly, he made a career on verismo and Puccini mainly:
Mario Diaz - Adriana Lecouvreur
This one is painful to listen to, effortful and flat:
Di rigori armato - Mario Diaz
And here is the masterclass, in which he talks about tension at the beginning... Am I missing something here??
Master Class by Mario Diaz, Professor of the Austrian Conservatory "Mozarteum". V1 Battle 07.11.2019
Notice that his speaking voice is lighter, and definitely more beautiful than the way he used it in singing.