While the counter on its own is a one time thing, having this trigger at the beginning of combat and then the ability to get that creature back with haste for that same combat makes a very tight window for opponents to deal with your shenanigans
I don't expect this card to become a breakout sensation, but its power will likely mean it commands a price of at least $2.00 sometime in the nearish future
Gandalf the White Gandalf the White is a nice addition to both Teysa Karlov and Elesh Norn, Mother of Machines decks, but I think theres a decent build of this Mithrandir as a commander of the kind of deck where you run Teshar, Ancestor's Apostle
Its not only a great mana rock, but also another fun way to get card advantage
Umbra armor won't do anything because it won't have to
One of the more common situations is your mana rock doubling up as a combat participant