Powder coatings, like liquid paints, can be formulated to provide a fairly wide range of properties. Picking any single powder coating formula as being "typical" would be a challenge in itself. Having said that, however, I would agree that in general, liquid epoxies can be formulated to have equal hardness to "typical" polyester powder coatings. (Pencil harness of typical polyester powders generally runs in the H to 2H range, with a few types running as high as 3H or 4H.) lf powder coatings have an advantage in mechanical properties over liquid coatings, it would probably be in their balance of properties, rather than any one single property.
Some paints are formulated to be extremely hard, but shatter like glass when subjected to impact. Other paints are extremely flexible, but scratch and mar easily or have poor abrasion resistance. I think the reason that powder coatings are regarded as being very durable has more to do with their combination of properties (good hardness, flexibility, abrasion resistance, and adhesion), plus the ability to achieve relatively high film builds, rather than being far superior in any one single attribute. Most successful coatings are sort of a balancing act, requiring good performance in several attributes rather than exceptional performance in one. Liquid 2K epoxies or 2K urethanes are generally pretty high performance coatings in most respects, and should probably be considered generally at the top of the coatings ladder.
I hope these comments help.