A way to protect yourself for an entire turn and act as a colorless draw engine is just too good
But the more aggressive the format is, the worse this looks, to the point where it might end up being unplayable
Much like Contraband Livestock, I think theres some cool gameplay value in the mini-game rolling this each upkeep
If an effect exiles Summon: Esper Maduin and then instructs you to return it to the battlefield, it remains face up in exile (unless that effect instructs you to put it onto the battlefield transformed, in which case it returns as Summon: Esper Maduin)
Like the other Glimmer creatures in this set, Enduring Tenacity returns as an enchantment after it dies, turning into an enchantment while keeping the ability which is more valuable than its power or toughness
However, its important to remember just how often creatures are going to die in a game of commander