Everyone talking about how it could be usefull as item repair are forgetting the other restriction. Even at level 10 it still has a restriction of 10 uses in an hour. Which means as a reload type of use, it could only reload 10 times per hour. If you're taking care of a party's equipment for example, the 10 second rule makes it unusable to long term items and heving a guy that replenishes 10 potions per hour is... arguably worse then just buying the extra potions, unless they're super expensive in this world.
If you think about it, when he says he had the 17th slime, assuming his dagger gets fucked up everytime he kills one... IF the skill usage was the same as in levl 10, that took him at least an hour just waiting for the cooldown even if he killed the slimes fast.
after level 10... i think what makes it broken is that he can use it on himself. Haven't read anything but these two chapters, but.... the only limiter he has is the 10 times per hour, but he can restore himself out of that, which makes him capable of restoring others out of their usage limits as well. MP is not an issue, HP is not an issue, if he can use restoration fast enough his party becomes immortal and resouce-free. The ranged weapon example becomes dumb, since the skill has no cast time, if he spams on himself and the gun, any gun becomes fully automatic. A flintlock pistol can be restored to when it was about to shoot, and it becomes a gatling xD. I guess unless he's insta killed he's unbeatable, and if sora gets goot enough to deflect anything it just becomes dumb. Well, most stories don't actually use the full potential of stuff like this, so i don't expect to see him spamming the skill in combat, even though theoretically he could, since any exhaustion on himself can be instantly reverted.
Oh, he mentioned being able to become young again, so... yeah. he can be fully immortal too.