Yoga is simple, a human being is complex. Nirvana or enlightenment is nowhere else but right within. In fact, that is our natural state. Then, why we don’t have it? With our indulgent perception and colored thinking we weave a thick web of ego-centric life that is forced outward - away from what we naturally are.
Associating “Ashtanga” to asanas alone is a disservice to the great eight-fold yoga path. Asana is one, other 3 secondary means are prana control, yama - behavioral commandments, and niyama - rules of living. Togther they clean much of the ego-centric web and create an anchor within, called pratyahara. Then the ability to concenrate becomes dharana, to meditate becomes dhyana and to contemplate becomes samadhi. Thus, one achieves enlightenment. Each of these stages is very complex when one is new, and very simple when one becomes an adept.
Yoga is not ‘just a stretching meditation’ whatever is meant by that. Coming to specifics, both flow and alignment are valid approaches to asana. Alignment is important because eventually that will synchronize the physical body and the subtle body to such an extent that awareness will be gently transferred from physical to subtle. While flow will teach how to conserve your energy when alignment is achieved so that you can sustain that state and make it ready for meditative exercises.
Yoga is not a tool or a technique. It is a journey. It is very simple, if you are in it for some leisurely strolling. It is very complex, if you want to run a marathon.