I've run the Urzaplex in a competitive red pauper deck, and the only issue I see is that you don't have any low cost creatures to hide behind while you get all three Urzas. I found that adding a few creatures with an X cost or another mana-variable cost helped out as you can cast them cheap as 1/1s if you get stuck, or as 10/10s if you draw them late. These also make great hate magnets.