InEffect: there are 3 main problems with sensei.
1) He's buggy AF with abilities vanishing and appearing as they please.
2) He doesn't do anything too special. Most of things he provides you don't need by midgame.
3) He can't really ramp the damage, so he's just a beefy object on the field.
Sure, considering game itself is kinda poorly balanced, no wonder most classes, spells, items, feats, etc are far weaker than others and could be skipped as if they never existed. Not to mention bugs.
I like Sensei for "endless dungeon" DLC, as he saves some time (both with buffs and fast movement) and can provide skill bonuses just like bard. Of course, compared to Sword Saints with Fauchard, his DPS contribution is negligible.
At least, beefy object is better than dead object, right?