I'm from India and during the summer holidays, I was sent to a 'Yoga Retreat' by my parents where we spent the whole day doing various Yoga and Meditation activities. This is where I learned Pranayama. Wim Hof's breathing techniques feel awfully simular to Pranayama (While Wim Hof has one technique, there are multiple techniques in pranayama).
Im a sceptical person and don't trust anything unless I see the benefits of any activity unless I see the change right before my eyes. For some reason I stuck to Pranayama and did it every day for a year or so. I observed a marked improvement in my lung capacity and my ability to hold breath underwater (I used to compete with my friends in the swimming pool and would always stay underwater for much longer than them). Also, whenever I'm in any stressful situation (like just before an exam or interview), I used to do the breathing exercise for a couple of seconds (breathe in through one nostril and exhale through the other and vice versa) and all my stress used to vanish and my mind would become very calm and sharp. Not sure why I stopped practicing it but I need to get back into it.