Maybe someone can educate me on this.
I have played plenty of games with both of those cards. I've beaten Cultist without buying it, and been beaten using Cultist by players who ignored it (rarely, but it's happened).
I have NEVER seen Rebuild lose to something other than Rebuild.
The issue I've had with Cultist when it has been beatable was that the deck it builds doesn't do all that much once the ruins are out. You probably need a few more silvers/golds to play big money with any effectiveness, and if your opponent was cleaning up the ruins and building something better, aren't you a little behind by that point (it also has the lulzy edge case counters of Vineyard, Gardens, Obelisk on the Ruins pile but I'm not even talking about those).
What am I doing wrong with Cultist that I don't see it as being as effective as you guys do?