First, Rings or Cloaks of Protection are good for everyone, and good for Monks no less, but Bracers of Defense are great for a Monk above probably all other classes.
I want to throw an obligatory complaint.
The Bracer's of Defense are horribly designed and should be redone, with two changes. 1) Remove Attunement, 2) Make them uncommon (I'm willing to budge on this).
Here is my reasoning. They are just a shield. Literally, that is their entire function, +2 AC, just like a shield. However, they do not stack with any shield or any armor. So, who can use them?
Well, there are only four classes that can possibly justify them. Wizards, Sorcerers, Monks and Barbarians.
Barbarians are both the most powerful choice for them, but also the most limited. Because wearing armor for a barbarian is a legitimate choice, until they have a 20 Con and 16 Dex, and even then, they can choose to use a shield, meaning the only use for the bracers is to allow them to also use a two-handed weapon. And there is a shield for that, the Animated Shield, which can be used by any class for the same benefit.
But, Monks, Sorcerers and Wizards can't use armor and shields, so this is a solid choice for an AC buff... but for half the AC, they can get a +1 to all saves with an uncommon Cloak of Protection.
Additionally, a +2 shield or +2 Armor gives the same AC boost to heavily armored people who don't struggle with their AC... and those don't require attunement.
No matter how I approach it, they aren't worth an attunement slot for their effect. The only worry is a high stat barbarian being able to get a +2 AC while weilding a 2-handed weapon, and that is fairly niche.