[TheCommonNoob]

For a promoted class that theoretically relies on a lot of combo play, Kensei's require way too many skill points to invest in.
(This also applies to a lot of other classes but I'm focused on Kensei for my argument).

Even if you're a destiny Duelist, you'll still have to make quite a few sacrifices while choosing your skillset to fit your already restricted playstyle.
All of Kensei's skills/passives together is 81 SP. On a destined non-human, no trait SP you have 50 SP to spend. (35 for non-destiny).

Out of those 50 SP, 13 SP goes towards often mandatory passives and skills, so you're left with 37 SP. (22 for non-destiny).

Now to proc those passives, you need to hit someone with the weapon skills associated but you also want to actually do damage when you hit someone with it.
So that's about 25 SP (Kagekiri, Toiken, Hirazuki, Raijinken, Sharenzan) spent towards your offensive skills to make sure they do good damage. Now you have 12 SP left.
(You don't even have enough as a non-destiny so you're already sacrificing something here, not even counting the other skills/passives).

Continuing on, Wazabane can live with 1 SP but it'd be really nice to have 5 SP in it for the bonus +10 Hit/Crit. So you have either 12 SP or 9 SP to divide amongst 10 really unique skills/passives.
(Sheathe Sword, Absolute Death, Absolute Fear, Absolute Pace, Sacred Art, Blade Barrier, Tekagen, Hidden Cut, Reflect Shots, Counter Edge).

Out of those 10, I would even argue that Sheathe Sword, Hidden Cut and Sacred Art are mandatory for Kensei to take.
However that concludes my example of a destiny Kensei having problems even with the extra SP. (It's way worse without destiny).

Firebird and Ghost also run into a similar problem with 67 SP total for Firebird and 74 SP total for Ghost.
Though they have a much less "mandatory" skill selection, they still have some like:

Firebird: Falcon Strike (5), Any Defensive Option (3), Roaring Falcon (3), Riser (1), Ignite Power (3), Magical Feathers (2), Plume Echo (Main Class Only, 1), Boon of Apus (1).
18 or 19 Total SP "mandatory" spending.
(Falcon Strike (one of the best offensive skills in the game), Defensive Option, Roaring Falcon and Riser for Boon of Apus to proc for feathers. Ignite power for good damage).

Ghost:  Ether Invitation (5), Scarlet Twister (5), Blood Spike (5), Wraithguard (Main Class Only, 3), Last Chance (2), Afflicted Spectre (2), Claret Call (1), Rising Game (5).
25 or 28 Total SP "mandatory" spending.
(Ether Invitation can be lower but that dramatically increases FP costs. Scarlet Twister and Bloodspike are too good to pass up. Then Wraithguard and the passives are what make Ghost, Ghost).

And now for example of another class and its promotions, a destiny Martial Artist can get just about everything they want from Martial Artist, Boxer and Monk.
A non-human Verglas (arguably one of the best reworked classes) without the Spiritual Fighter trait can get everything they need just from destiny SP.

Another good example would be the reworked Black Knight.
Even with only 35 SP, 36 SP (with the Mercenary Work trait) or 38 SP (as a human with the trait), you can get about everything you need to thrive with Black Knight being in the main class or sub class slots.
(Without even being restricted to a certain weapon archetype like Firebird or Kensei).

And as an extreme example, Rune Magician, actually one of the best classes in the game only needs 37 SP to be fully kitted out. Sure you "need" to main class it but, if you're going to be a Rune Magician you might as well have anyways. So any race with the Arcanic Study trait can just beyblade and let it rip.

To end off my rant though, please take a look over some slightly older classes and promotions and reduce the SP cost of some skills and/or passives (especially Kensei)! 
It's hard to be thematic when even destined, you aren't the master of your own craft.