First, lets look at Hydreigon's base stats. It has 105 base attack, 90 base defense, 125 base special attack, 90 base special defense, and 98 base speed. It has a stat total of 600. So, Hydreigon's growth rate is slow, and like you said, it does evolve from Zweilous at level 64, which can be a pain because of its growth rate. Just like one of the previous comments said, its movepool contains normal, dark, and dragon type moves, but there are TMs and HMs as well. Hydreigon's stats are great and can be useful for the Elite Four as well. So overall, even though it's growth rate is slow, and leveling up requires a lot of patience, in the end, it can be a very powerful and useful Pokemon to use. So, I would say it is worth it.
I hope this helped!