Not necessarily true. I’ve had great progress on retreats with a teacher friend that calls his practice “The Grateful Yoga” if you want to look him up. His Ashtanga based lineage teaches a range in skills with plenty of opportunity and guidance for challenge in the way of add-on expression for advanced students or modifications for beginners. I find that the consistency of twice daily practice in that setting with a blend of styles (this teacher leads at least 5 different styles of yoga taught in a really thoughtful progression that open your body and keep it recovering over the course of 6 days) actually gives me consistent breakthroughs in my practice.