Here's how I'm seeing it.
Open Hand Monk vs Kensei 5th level (we'll assume +5 dex, no feats for now)
Open hand makes 4 attacks, 2 with staff, 2 with flurry, for 2d8+2d6+20. They also get the Open hand optional effects with the Flurry of Blows attacks
Kensei using a greatsword/maul makes 2 attacks and pummels, for 4d6+10+1d4
Add feats, Open Hand stays the same
Kensei uses GWM and we'll assume hits 4d6+30+1d4 clearly taking the lead.
Polearm is even better potentially, because you can replace the bonus attack.
Now let's jump to level 17, no feats
Open Hand can flurry for 4d10+20
Kensei 4d6+16+1d4
GWM stills wins out though, getting 4d6+36+1d4
Polearm is actually worse, unless you also have GWM.
So, the only way Kensei gets better damage on a burst is to grab Great Weapon, otherwise the other monks end up with better damage over all, and if you've taken this sub-class I'd assume your intentions are to get better damage. And in fact, the only wayto out compete the monks normal non-ki spending damage is to use a weapon that has a 1d10 or higher damage die, because monks generally can use 1d8 weapons, meaning a longsword wielding monk is just as good as a staff monk, unless they are using two hands, until the point where the bonus attack from martial arts beats out the 1d4 from pummel... which is really early in the monks career, as I will gladly go for 1d6+mod on a roll before a 1d4 on a roll (because the weapon attack still has to hit)
The Kensei's only advantage for flexibility seems to come from shortsword wielding, and it only gives an AC bonus, which I have never seen be an issue with monks. Every monk I have ever DM'd for very quickly had a AC of 18 or higher, and the ability to Dodge as a bonus if they felt threatened.
So, you lose out on damage unless you take two of the most often complained about feats in the game, and you don't really seem to gain much beyond accuracy on hitting and adding to your armor class.