Not being able to wear anything in their head slots (except Ribbons for Female Monks, but if you're using a Female Monk there's already an issue there) is actually pretty crippling for Monks. I'm not even sure if the ability to be the only class to stack Martial Arts and Attack Up or Concentrate makes up for that. For any other class, Punch Art is terrible without Martial Arts equipped, so that locks up your Support slot and is one of those things that makes it not a great skillset (especially since a lot of Punch Arts lost their Unevadable property from Vanilla), though to be fair, a lot of the Physical tree don't have very good skillsets.

There's a bit of an issue with trying to improve Heavy Armors by giving them Elemental Resistances and Status Defense, the latter of which you can probably argue is already in place with the rarer items like Grand Helmet. First - most of the enemies don't use Elemental attacks in the late game when Heavy Armor falls off - they generally kill you with physical damage instead.

Second, ironically the fact that the AI is smart enough to not use Status Effects on targets that are immune hurts the viability of gear that aren't stat sticks, as it means that you need to block every single dangerous status like Ribbons do to protect yourself and then the enemy will probably kill you with a Physical that takes off your entire bar in one shot anyway. This also applies to Elements to an extent, so things like Elements tend to really only matter if you have a party set up to take advantage of it (such as Asura Flame Shield absorption parties)