The boots, gloves and helmets are oddly rare and expensive compared to the price of a full armor.
I would remove the Crit Immunity property from a mundane helmet without blinking. It's absurdly powerful and a critical hit is a much broader concept than just a hit to the head.
Heavy armor parts do look ridiculous worn over robes so I hope it won't be possible for Wizards to wear full helmets just because good bonuses. Magical helmets that don't require armor proficiencies could be much lighter by design so they would match both robes and full plate alike without looking out of place.
