There is maybe one monk subclass that is a trap to use.
Drunken Master makes FoB also disengage and gives a solid reaction ability.
Open Hand gives secondary effects to FoB for no extra cost, and a solid late level ability.
Sun Soul gives increased range, ant-will radiant damage, and some spells at a good ki cost. Even if you never throw a radiant fireball, the level 3 makes it an improvement on not having a subclass.
Astral whatever and dragon guy I don’t care about enough to even reread for a review.
Mercy is just good. Occasional ability that costs more than it should, or does less than it should, mostly on the healing side, but action economy is solid and it’s very fun, and the subclass adds a lot to the character.
Shadow needs better action economy, and bring the teleport at level 3, adding the advantage clause at level 6, but overall it’s still solid.