It does have a downside in that it stops being a creature if an opponent casts a creature spell
White decks can easily flood the field with creatures, allowing for an aggressive monowhite deck to become part of the metagame
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Its better than Awaken the Sky Tyrant, but thats damnation by faint praise considering so are some of the commons in this set
The more the lands untap, the more mana can be floated to feed its abilities
Take Kalain, Reclusive Painter, for example, who is a Black and Red 1/2 that creates a Treasure token upon entering the battlefield, but also reads, Other creatures you control enter the battlefield with an additional +1/+1 counter on them for each mana from a Treasure spent to cast them. Not only does that mean you get an extra mana, but that its now in your interest to make as many Treasure tokens as possible