I cannot say much to the Paladin as I don't like the class, but on a Fighter or Barbarian the Polearm Master/Sentinel combination is not more "broken" than Fighter or Barbarian with GWM + Lucky or a Fighter with Sharpshooter + Lucky. You get more control but less damage, for me that's ok. Seems to be a Paladin "problem" because you can have the polearm + a lot of damage.
But I don't see it really as a problem. Haste giving not just one additional attack or the vulnerability from Wet is far higher on that list.
