This is true far too much emphasis is put on certification and being Yoga Alliance registered. Are there no Yogis in the world uncertified that are not experts and great teachers and practitioners? Of course there are many!
Though it is good to have a 'standard' of quality at least if someone is 200/500 hours you know they have good knowledge of Yoga - though does not necessitate they are great teachers.
Your example of going to seminars/workshops with people, someone may have done 200+ hours with many different leading teachers in that way but not be fully certified or registered with Yoga Alliance or another similar organisation. In martial arts for example, I know many that have very high standard and great teachers but have no certificates or black belts at all - they studied with seminars, classes, home study, etc. Not forgetting Bruce Lee! - no black belt or grade, no certification, yet nobody would doubt he was an undispituted Master.
I think with Yoga you either can do it or you can't, if a 500hour registered Instructor (in perfect health) can not do any remotely advanced exercises, you would have to question their skill & knowledge.
Many places such as gyms employing Instructors will do a check on certification , in the U.S. some will look specifically for someone to be Yoga Alliance registered, though again it is not the be all and end all of Yoga having a nice certificate - variations in teacher training courses and standard is huge!, some you basically turn up and get your certificate, others will really test you and not certify you unless you are good enough.