So, now that they clarified that Kensai weapons ARE monk weapons ...
Kinda. Here's the tweets from Mearls:
@mikemearls Dec 13 Quick UA clarification - kensai weapons are not monk weapons
@mikemearls 15 hours ago A better explanation - a kensai weapon is not automatically a monk weapon. This is in response to a question about the shortsword
BUT
@mikemearls 15h hours ago Sometimes knowing the system guts is a bad thing. You can get the benefits from Martial Arts when holding a kensai weapon.
Of course, this is really not much of a distinction, since Martial Arts is the only Monk feature that even mentions Monk Weapons.
And to add to the confusion:
@mikemearls Dec 13 mainly balance - can't flurry with kensai weapon
but I think he was mis-remembering, thinking you could flurry with monk weapons. Flurry is Unarmed Strike only. It's the
damage an unarmed strike would do that's dependent on the Martial Arts, and since you get all those benefits, there's nothing preventing you from using a kensei weapon and then a bonus action FoB using full Martial Arts damage.
I am still dubious about it being worth it, except at early levels. As early as level 5 you'll be able to do one or two great sword attacks and one or two unarmed (pretty strong) OR two great sword and pummel for extra 1d4 damage. This is so subpar in comparison I am not sure why it's there.
Something needs to be done so the heavy two-handed weapons aren't the obvious choice for kensei, but just an option. For a fighter, you're balancing with giving up a shield.
Pummel could be useful if your kensai weapon is a bow, in which case, when you fight at range, you will not always be able to punch things nearby.
Per Jeremy Crawford, that's not meant to be an option:
@JeremyECrawford Dec 12 Rereading a playtest rule, I'm chuckling at the absurdity caused by accidentally omitting "within 5 feet of you." Pummeling at long range!
Could a polearm master build bring something to the table?
Reach?
Really, in the end, I think these statements from the wotc staff imply this version of the Kensei tradition is dead in the water:
@JeremyECrawford 18 hours ago It was an experiment. Playtest feedback is rapidly showing it isn't an experiment worth carrying forward.
@JeremyECrawford 19 hours ago It's funny you ask. We already have a different version of the kensei in mind. That's the nature of design: iterate, iterate, iterate.
@mikemearls 14 hours ago All of the UA stuff will receive revisions - honestly, monk is just sloppiness on my part.