I think they tend to be low-agency for the caster, but some factors may swing them up to being high-agency.
Let's think of Armadillo Cloak. In situations in which you are under pressure and have to drop it to gain life and save yourself... well, you have to do it, regardless of how risky it is.
But there's a lot of nuance with the environment, particularly with the speed:
- Heroic deck in an environment with sorcery speed removal: Low agency. It's just too good to put it on
Fabled Hero, even if it dies next turn you got in effectively 10 points of lifelink damage.
- WG Aggro T3: High agency. Turn 3, you are getting some damage in, and you can opt to play a 2-drop or Armadillo Cloak. Depending on the opponent's deck and signals, you may want to take the risk and put it on
Watchwolf. Or you should just cast the 2-drop, hedge against removal and draw it out of your opponent's hand.
- UGw Midrange in a slow environment: High agency. You are again trying to find a good window to cast it. You might sandbag it until you have a
Dive Down,
Snakeskin Veil, or
Counterspell. From an empty board, you can flash in something EOT then untap and go for the Cloak for a surprise Baneslayer. Cloak gives you decisions of risk of CA loss vs tempo.
- UGw Midrange in a fast environment: Low agency. You can't afford to make a low tempo play, so your best bet is to go for it.
From the opponent's PoV, it's low-agency, since you always want to kill the enchanted creature. But that depends a bit on power level and density of removal. The presence of auras in an environment might make saving removal and keeping mana up at the right times a good idea. It's only a valid decision though if 1. The density of removal isn't high enough that you have to pick targets; 2. The power level is low enough that a threat only becomes "worthy" of removal after getting enchanted with Cloak.
So generally, I believe Armadillo Cloak is low-agency both for you and for the opponent. It may become high agency, though, in very slow formats.
Most good Auras have a bunch of power built-in to make up for the risk, and that's why I like equipment and Runes. They fix this problem, still giving the choice to decide what to equip/enchant and when, with less of a cliff in power between the scenarios of removal/no removal. The game is less about them and more about other choices/risks as well.
