I've been wanting a Kensei since the playtest of 5th edition. Yet I find myself completely unimpressed.
First of all, being able to use any weapon as a dexterity weapon may be too powerful, but part of the problem is the new terminology.
This martial weapon is a Kensei Weapon, not a monk weapon, does this mean you don't get any of the benefits of Martial Arts while wielding it? If it does, then all of your unarmed strike abilities are worthless, because you are hitting with str+mod and 1 damage.
If the Kensei Weapon is a monk weapon, then pummel is nearly useless, as it is only +1d4 damage, unmodified by anything. The only advantage of it is using your bonus action to automatically hit, but by higher levels you could be hitting for 1d8+5 instead of a flat 1d4.
If it isn't a Monk Weapon, then it also makes the ability to get +2 AC nearly worhtless, because again, attacking with unarmed strikes is a very bad idea when you lose all your normal bonuses.
So, the advantage is getting a 1d10 polearm or one of the heavy weapons like the greataxe and using them with Finesse as a monk. After all, the staff already did 1d8, so you need to be looking for a 1d10 or higher weapon die to increase your damage.
Then Precise Strike is... not useless, but really uninspiring to me. A +3 to +6 to hit with 1 attack, at the cost of your bonus action, when your bonus action is so vital to the monk's flexibility. It just feels like too little, and the magic weapons a fist monk already has.
Sharpen Blade is very nice, getting you a solid benefit for a chunk of ki, and Unerring Accuracy isn't bad, but they come a bit late to make this feel like a good sub-class.
Actually, Sharpen Blade is really good, on a second reading. You can "bless" any weapon, even a magical one. So if you are wielding a magic sword you can make it an even better magical sword.
Overall though, it doesn't seem to work for me.
Tranquility is amazing though.
You start by getting a long term, almost at will Sanctuary spell and a massive pool of healing. I get that it is double the Paladin Lay on Hands, but Paladins are definetly more damage oriented and have other spells on top of their smites, so I'm okay with this being a larger pool. And you can use it as part of your Flurry of Blows giving you even more options.
Emissary and Douses are great for roleplaying peaceful meetings, and make it easier for hot-headed enemies to sit and talk with you. There are a lot of limits on Douses, but it is a cool, if niche, ability
Anger of a Gentle Soul is crazy good. By this level most monks can easily make 4 attacks a round, making this an additional 68 damage against a tough foe. Of course, to pull that off you have to choose not to heal the ally, so it will generally be 3 attacks and a use of Healing hands.
Yeah, I really Like tranquility. It is very difficult to play a pacificist character in D&D, but this character isn't penalized for fighting. They just are much better at negotiating peace. I think it is a really solid design