Is it worth to unite HRE? I just have to click "unite HRE button" and its done but is it not better to stay with 35 vassals? If i understand right after i unite HRE i lose every emperor and reform benefits so that s why im thinkink not to do that. 
Uniting the HRE will indeed remove all emperor bonuses. I wouldn't unite it myself. Uniting will make you much less powerful overall.
Less powerful = the most powerful nation in the world that can rotflstomp everyone without any effort anyway.
It also has a side effect: the game can quickly become boring...
Well i just United HRE. My land forcelimit is 280k, my manpower is 320k, and income more than 130gold/month. I started this game as Brandenburg so my army is invincible. Ok what now? I probably wait for expansion pack to next month cuz whole my motivation to continue HRE is gone.