At level 3 you have 2 runes. That's only 2 'invoke rune' ability until you take a short rest, plus 2 Giant might a day.
That's a little better than a Warlock's Spell Slot, but without all the cantrips. Sure, you got heavy armor and weapon proficiency, but you're also wading in melee where you're expected to be hit, not slinging Eldritch Blast from the next county. Nor do you get stuff similar to Eldritch Invocations that give you at-will spells.
I think what you could do, to test the water, is only allow 1 'invoke rune' action per short rest. If nothing else, I wouldn't allow someone to stack them, even if they have multiple runes and they probably shouldn't stack with Giant's Might. It's one or the other.
Also, you need to decide is the passive ability can be passed on to another character. That's kinda important. If I put my rune on my buddy's weapon or shield for exemple.
... This makes me realize that, as written, you could technically encribe the same run more than once on your gear and thus get to use its invocation more than once. Huh... neat!
EDIT: Ah, no, nevermind, it specifies 'A different rune in each object'.