Haku is the weakest ★6 unit that is currently available in the game. The long range unit Gaara ★6 even has more attack. He has a good amount of HP (nearly the best @ 1428).
His Jutsu and Secret technique are also both AOE making him quite useful for certain situations. With taking into account any modifiers his normal Jutsu does 2223 damage to each unit he hits and his secret technique does 4979 to each unit hit. Previously was not considered a great unit and quite a few rerolled accounts for a better ★6 but he has found a use in PC due to the AOE Jutsu. His Jutsu costs 5 chakra to use so have to wait until his third turn to be able to use but if used correctly can win you that match. Haku does have his uses but other 6*s are preferred and potentially will become obsolete quite quickly.
AOE Jutsu can make him useful for PC
Jutsu can get ultra combo (15 hit or more) when two enemies in range and gives you bonus chakra
Very High HP
Lowest ATK 6* currently in the game make others a better choice
5 Chakra Jutsu means that you have to wait a little while to be able to use"
Thanks to bfw182 for his awesome evaluation notes!