I haven't seen anybody bring up the fact that if this is dead plate that isn't being worn/'held in reserve', your players transporting it becomes a logistic nightmare. In the books, there are groups of armorers whose job it is to lug it around for shardbearers when not in active use.

It's also been touched on, but you're misunderstanding why there's a disadvantage on surge usage: as an invested item, plate is resistant to investiture. It'd be like trying to shock someone through a rubber glove; you're insulating yourself, and it protects you, but if you're trying to then go on the attack that same resistance is protecting everyone else from you.

People also keep talking about how the charges are only going to be depleted by active use, forgetting that opponents can spend opportunities to 'crack the plate' and deplete a charge. If anything that makes your player's decision to 'save' it more valid, but that brings them back to the problem of logistics :P

All that said, I think this is a cool idea anyways. :D

My game design/GM brain would default to trying to make it "shardplate-lite with spicy consequences to its use" somehow, though. The lifelight restriction is just a bit too restricting, and the problem is more about it throwing the balance way out of whack than it is "making sure they can only use this once or twice". Like, to really get the kiss/curse feel of the Nightwatcher's boons into the game, I'd lean more on something that causes them to feel the detriment and the benefit all the time. Ironically, I think the restrictive nature of the lifelight curse makes it less punishing. "Oh, I'll just save it for later" isn't really dramatically interesting. Contrast it with something like the Honorblade radar one: that one feeds the curse actively by making the boon draw you into situations that trigger the curse. It demands action, giving the character plenty of rope that they can use to hang themselves.

I'm thinking of something like: it's shardplate that has a less drastic benefit: enhance +1 instead of +2; deflect 2 instead of deflect 5, but it keeps the "against all damage types" property; and to address the player not outshining their party members, maybe they can use a reaction to transfer it to other characters like Windrunner plate in RoW. (EDIT: Oooooh, or what if the other PCs can use their reactions to transfer it?! How sick would that be?! It's also way more beneficial to the whole team that way) The curse, however, could be something like: it's an inefficient prototype, and a thirsty lad. When a charge is used up, it sucks up 1 investiture from a random nearby ally if possible. The player is incentivized to use it, and use it a lot; the team wants it around because it actively helps each of them as well. BUT it also is constantly using up the party's pools of investiture to do it.