I don’t know about JASO, but I believe the European ACEA specs are self certified, which destroys the argument that “they meet it, but don’t want to pay for it”.
It does leave things open to people not playing fair.
However there have been times when one big oil company tested another big oil company’s product and found it wanting. After checking their results they went public with it not making the standard. I believe this happened to Mobil once. You also have PQIA in the US performing regular tests, and issuing warnings if they are concerned. So the big companies tend to watch each other.
This is why many stick to name brands they trust, these big companies have too much to lose by cutting corners. Market share and name brand recognition are big drivers.
I’ve heard from oil industry people that hang out here, that both Shell and Castrol are very careful in making sure their products meet their claims. I’m sure the others do to, but they are they companies I have heard more directly about. To add, I would have no problem using Mobil or Valvoline either.