Doubling your combat steps is a dangerous ability that can be broken quite easily, especially in a set with Isshin, Two Heavens as One
To bolster monocolor decks, what if that resource included cards of a given color
The Indestructible helps to keep it around, which is a big boon after you've invested mana and/or dorks into getting it onto the battlefield
"[SLD] More Hatsune Miku Commander Deck Cards"
While its ability to endlessly regenerate itself is one thing, being able to mill six cards from the top of your library instead of drawing every turn is just too powerful of an effect for players to be able to access for free
I just think the card is well executed it being an artifact plays well with Urzas Saga, Improvise and Springleaf Drum, and unlike Thalia, Guardian of Thraben where youre locked out by the tax, you have the choice with Esper Sentinel which presents interesting in-game decisions as you have to balance the tax on the mana versus providing your opponent with card advantage a similar decision to that of being on the play or on the draw
