This can wipe out most creatures within the first few rounds of play, barring any defensive maneuvers like instant buffs or counterspells
Casting a spell for its cleave cost doesn't change the spell's mana value
You also need to make sure you hit your land drops, either with draw effects (Qiqirn Merchant and Adventurers Airship provide usable bodies as well as draw smoothing) or cards that directly give you lands or mana (World Map, landcyclers, Blitzball)
Overall, Verhey says, "this card is extremely comparable to Plating on power level." In the article, the designer takes pains to emphasize that pre-banning cards is not the new normal - for Pauper or any other MTG format
That coupled with Ward make Webby an indispensable card, and being able to shift for such a low cost can be helpful in certain situations, too.
Not to mention, the card's own recursion is practically built in thanks to its use of the escape mechanic also seen on Underworld Breach