There is no better collection of exercises for the ongoing health of your body than Hatha Yoga; there is also no set of exercises that will rip your body to shreds and completely aggravate your existing injuries and asymmetries more dramatically than misapplied Hatha Yoga asanas/physical poses.
A rational, sequential program for prehabilitation and rehabilitation features many Hatha Yoga practices and asanas, but the timing for the introduction of these practices and asanas is absolutely critical.
Would doing a full forward bend asana exacerbate an existing sciatica nerve disorder? You bet it will...and the further aggravation will send searing pain down the leg.
Is doing a full forward bend asana a good idea for someone who is recovery from a sciatic condition and is now pain-free but very stiff and relatively inflexible? Yes, it is a great idea, but it has to do been done in very non-challenging stages to allow the body to acclimate to the physical stress.
In any rational, sequential exercise program, the following thought should prevail:
INCREMENTAL INCREASES OF PHYSICAL STRENGTH LEAD TO POWER;
DRAMATIC INCREASES OF PHYSICAL STRENGTH LEAD TO INJURY.
The same Hatha Yoga exercises introduced at two different times for the same condition have wildly varying effects...and yet people will chant robotically that Hatha Yoga is great for all injuries. This kind of gross oversimplification leads people to further injury and a relative frustration with Hatha Yoga as a discipline.
In more traditional Yoga learning environments in India, very basic contraction exercises that stimulate the nervous and muscular systems are usually taught to students prior to moving on to more challenging Yoga Asanas. The Pawanmuktasana Series, "The Wind Releasing Series", is taught as a foundation practice to learn the skills necessary for the further perfection of more challenging yogic asanas.
In Sanskrit, this practice are referred to as Sukshma Vyama, which means "subtle exercise". It is an incredibly useful practice because it opens up all of the limbs, joints, and muscles of the body so you can be successful at the more advanced Hatha Yoga practices. The group of exercises removes blockages preventing the free flow of energy in the body by specifically targeting the nervous system.
Rarely will these practices be taught anywhere in the USA, as the usual situation at a Yoga studio is to let students dive-bomb into whatever class sounds good at that moment and let the dust settle where it may. Students begin to think there is something genetically wrong with them, i.e. "I guess I am an inflexible person", when success is really matter of stretching technology and practices being introduced at the right times.
Most of the Yoga teachers in the USA have either a dance or gymnastic background and were, therefore, already quite flexible from previous training before coming to Yoga. Dancers and gymnasts who begin their activities early in life achieve incredibly flexibility without the sorts of accumulated scar-tissue adhesions and peripheral nervous system blockages hindering their success. When they teach later in life, there is a sort of bewilderment at a student's inability to open up into asanas.
The student's response is to "try harder" by putting more stress into the asanas, setting up the potential for injury.
The real answer is to have the student back up some: get the necessary body work to fix the scar-tissue adhesion and nerve dysfunction, start with the basic Hatha Yoga curriculum, and then learn real flexibility techniques that require the awareness to travel inside the body rather than straining against uncooperative nerve/muscle structures.