They are among the most stable pigments known. They are among the most stable synthetic organic pigments known, yes. Put them in a contest with pure carbon, and I bet the carbon would win. Put them in a heat test against cobalt blue, and I would think cobalt would win. That was my point.
Reasonable permanence is acceptable.
And, many people will not use Cobalt Yellow, because they don't consider it permanent enough. For them it's ASTM I or nothing. For some, the ASTM ratings are not even strong enough. I believe that many ASTM I rated pigments are not suitable for outdoor use, because they will not last. The ASTM ratings are useful, but they are not omniscient.