Blue
Ravenous Bugblatter Beast of Traal
Do the polearms on a kensei outperform long bows (an allowed weapon)?
Yes, reach weapons are significantly better than a long bow. Much of it is how it interacts with other monk abilities, and opportunity options.
Until 17th level Monk they do more damage on every hit.
By being in melee they enable the option to make one attack an unarmed attack to use Agile Parry for +2 to AC as well as the monk bonus action attack. Being in melee also allows Flurry of Blows. Stunning Strike requires a melee attack.
While GWM and SS are a wash, there are other feats such as Sentinel available for melee weapons but not ranged weapons. (And a monk able to stop someone outside of their reach to prevent an attack on them is a nice ability.)