I guess it really depends on what kind of yoga instructor you are looking for.
I have a 200-RYT and two 16-hour yoga certifications (all three trainings were live instruction), but do not have the NESTA online certification. The NESTA certification appears to be all done through at-home study, essay, and exam. The reviews from people who have taken it appear to be reasonably good.
Here are my concerns.
Since it is online, there is no process whereby the “instructor” is watched to see if they are moving safely and properly in their own practice.
Also since it is online, there is no process to see whether the instructor – even if they can move safely and properly in their own practice – can assess individuals and cue them properly.
They make it clear on their website that it is “sports yoga.” There is a lot more to yoga than the stretch aspect. Are your clientele wanting only the stretch piece (in which case a fitness yoga certification like this might be totally fine), or do they want more of a traditional yoga, which focuses on the pose, the breath, and mindfulness aspects of yoga?
I wouldn’t automatically rule a candidate out if this was their only yoga certification. However, I would ask this person what their home practice looks like (how often, what kind, what have they learned?). I would also ask who they’re currently studying with. If they’re not studying with a teacher and / or practicing, I’d be wary.