I don't feel like Four Elements Monk is that bad.
The main issue is that Open Hand is so unbelievably strong that both other Monk subclasses are complete garbage in comparison.
Like, Open Hand gets the manifestations and Tavern Brawler to make their unarmed attacks hit like trucks with high Str and Wis (With also extra utility because of synergy with things like Radiant Orb procs).
While both other subclasses just have worse unarmed attacks due to lacking manifestations and often weapons are just inferior to fully buffed unarmed.
I feel that Four Elements is better than Shadow though. Fire Snake is a nice boost to damage output when dual wielding weapons (Or when using a 2 handed weapon with weaponized bonus actions). This is on top of a few extra spells like Hold Person, Fireball and Thunderwave which will scale with your attack stat (Str/Dex) unlike normal spells and such spells are also still decent even without upcasts.
One of the main things that Four Elements is missing... Is their Fourth Element. They only get Fire, Water and Air skills... Other than that, maybe some extra scaling on Fire Snake's buff might make it more comparable to Open Hand (Like if it was 1d4 + Wis mod damage to your attacks like a manifestation)
Shadow is the Monk subclass that needs the most help though. All it gets over Gloomstalker and/or Rogue is Shadow Step (With Shadow Strike being just complete garbage compared to simply using Flurry and the excess speed a Monk has) and considering most Monks multiclass Thief for the extra bonus action it's not really standing out on its own...
