It's a hugely playable card in the new Hunny archetype, and while it's not yet been tested at a competitive level, the design of the cards and mechanics should carry it through
If you want the official boundaries in one place, PrintMTGs proxy policy is very explicit: casual play and playtesting yes, sanctioned tournaments and resale as real cards no
When Veil of Summer was first printed, it had a big impact on the format, with people even calling for it to be banned
If your untap step is somehow skipped as your next turn begins, your phased-out permanents won't phase in until the next untap step you actually have, but you'll no longer have protection from everything and your life total can change again
For that reason, it is also a genre where the strength of a confirmed PSA 10 easily stands out
Unless youre a diehard collector, its unlikely youll come across many