All in all, restricting the game on uncommons and commons made these challenges more skill-testing (You need now to build your deck really properly and linear for a single challenge, you just cannot build a general deck and try to beat this challenges.) because of the absence of "free wins".
I don't think most of us don't want to build decks that are single-purpose / AI-defeating only. We'd prefer to be testing our best general-play decks against the AI.
I often want to test my decks on the AI challenges, but by default they disallow rares which cripples me when testing general purpose decks. And the normal AI is just too poor.
We need something in between. Can I suggest that we have AI challenges that allow toggling use of rares on/off? When rares are enabled, you get no gold reward for winning, of course.
P.S. I'm a games dev by profession, also experience with web including PHP, so happy to pitch in on dev sometime.