This can easily be exploited with untap effects like Arbor Elf, to produce a ton of mana in a single turn
it also finished second at Championship Finals Yokohama
The first ability resolves before the spell that caused it to trigger
For example, a card like Sandwurm Convergence says you cant be attacked by fliers, but that protection doesnt extend to your partner, so your opponents can just send all their flying attackers at your partner for the same effect
Basically, cards you can get as promos will also be available in main sets
Ive met almost no Elf decks who would decline this card, especially something like Ezuri, who could tap out for mana, activate twice, and then attack for a billion