Well, consider there is a cost associated with playing fastbound/exploration. It is costing you a card for the effect, so you are playing with 6 cards not 7. Also, each land you play is one less card that can be a spell. So the 4 land to 1 land on the play is certainly sweet, but it required you to have 4 lands in hand to do it. Even with the draw step, you only have 4 business spells (1 of which was exploration/fastbond), so really 3 business spells. two 2 drops and a 4 drop? That's certainly hitting the ground running, but you are now in top deck mode T3 forward. One sweeper or efficient blockers and you lose.
It's an explosive move, but it's far from steam rolling games in cube in my experience. Consider too that the power-max crowd generally doesn't run either card and I'd bet the feedback on both is that they don't do enough to justify a card outside some crazy combo deck (storm or whatever). Not saying you shouldn't run either card or consider them at least, but they are less broken than they appear IMHO.