Armor (plate), uncommon
This suit of armor is reinforced with adamantine, one of the hardest substances in existence. While you're wearing it, any critical hit against you becomes a normal hit.
Plate consists of shaped, interlocking metal plates to cover the entire body. A suit of plate includes gauntlets, heavy leather boots, a visored helmet, and thick layers of padding underneath the armor. Buckles and straps distribute the weight over the body.
Notes: Immunity: Critical Hits, Combat, Warding, Str 15 Required, Stealth Disadvantage
That feels a little weak for a full plate made of the best material you can find. Bonus to AC? Resistance to piercing damage? Crit fails from enney breaks their weapon? Idk, I just feel like it should have something more.
It says reinforced with, not made of. Also negating crits is bigger than you seem to think it is.
Adding onto this, many DMs including my own homebrew crits to deal max damage + normal damage to make them feel more impactful (bc otherwise a crit could do less damage than a normal hit that just rolled high, and thats BS) so negating crits becomes a much more big deal.