For reference, the 14-hp identities are She-Hulk, Thor, Hulk, Drax, Sp//dr, and Colossus. There is obvious overlap between high health heroes and heroes who want to attack almost every turn due to very high ATKs (She-Hulk, Drax), wanting to engage and kill minions (Thor), or wanting to smash (Hulk...see Hulk Smash). A stun status can basically ruin an entire turn, and Unshakable can really help.
Being steady essentially amounts to discarding the first, third, fifth, etc. status cards of the same type your hero gets. The next best alternatives are The Night Nurse, but only in protection, and Athletic Conditioning. I'd argue the Night Nurse is better if you are playing protection, but Athletic Conditioning is clearly worse as it is an event that must be played when you are stunned/confused, and Unshakable is proactive (and answers one of each status). Note that Thor gets Godlike Stamina which is way better than Conditioning but still arguably worse than Unshakable except for the healing.
If you want to play this card, check that your scenario actually has ways of stunning/confusing you, but if it does this is a game-changer for the eligible heroes!