I'm sure if Haku had gotten more training, he would have been very impressive in the future.

As it is, Haku was unable to defeat a person who was impervious to needles (Naruto's chakra shield and healing).

If Haku's opponent had some kind of shielding (as Gaara does), or was able to deflect the attacks (the Hyuuga family's Kaiten), Haku would be wasting his needles. Plus, Sasuke was able to predict Haku's movement using his incomplete sharingan. He was also protecting Naruto at the same times. I'm sure Kakashi (or Itachi) would be able to fully avoid any needle attacks.