So, I'm currently playing a Mercy monk, and as I previously mentioned, I think this subclass shows that someone at WotC gets monk, because Mercy seems intentionally designed to mitigate the class's mechanical weaknesses:
1. being resource starved
2. being overly reliant on bonus actions
3. having little to offer the party
4. having too few choices to make (aka stunning strike or bust)
Level Three
Hand of Mercy/Hand of Harm
Mercy's signature abilities are introduced right away, and crucially, there are two of them, so that players immediately have choices. Both work off Flurry of Blows or the unarmed strike, so there is no extra demand placed on the bonus action. Hand of healing costs no extra resources and can be used to either heal the monk or an ally, making the class more tanky without sacrificing as much offence as Patient Defence, and providing group utility - it is super fun punching someone else's character back into consciousness! There is additional offence (hand of harm) allowing you to mini-nova by combining this with FoB if you want to spend 2 ki.
Right away, Mercy monk has more multiple choices each round (while ki lasts): tank better, hit harder, or heal a buddy.
Level Six
Physician's Touch
This is where the Mercy monk really begins to shine, because it feels like the monk as you imagined it: you can attack and defend effectively, both at the same time (what a concept!). Additional features are stacked onto Hand of Mercy/Harm, so there is no extra resource cost. Including the ability to automatically give a target the poisoned condition via hand of harm (all their attacks are at disadvantage). With no saving throw. If you are tanking a BBEG, this is like getting to have Patient Defence without sacrificing any offence, except that it also protects the rest of your party. There are limitations - sucks if you are fighting a vampire or something - but that's fair.
Plus you can now punch blindness, deafness, poison, disease, paralyzation or even being stunned out of an ally. So...suddenly Stunning Strike isn't your automatic option. It's still a good option (and you could even stack it in there if you really want to blow through ki), but it feels great to have more than one choice to make.
Level Eleven
Flurry of Healing and Harm
You can now do two heals instead of one, if you like, and both Healing and Harm no longer cost ki. It's okay...what's with WotC always wanting to address the monk's resource scarcity at high levels when it is no longer much of a problem?
Level Seventeen
Hand of Ultimate Mercy
You can now punch your ally back to life, within 24 hours, along with 4d10+Wis HP, and freed of any of the conditions listed above. Like Raise Dead only way, way better - you could viably do this in the middle of battle, and it ain't gonna cost you 500 gold. I don't know if I'll play this character long enough to get to do this, but I really, really hope so.
What makes this subclass so great is that by level three all the pieces are in place, and by level six you have everything you really need. It's still resource dependent, but when burning through resources you feel like a superhero, rather than just adequate. Whoever designed Mercy understood the monk's limitations, and compensated brilliantly.
That person should be asked to lead the revision of the current UA monk.