I see threads like this and I shake my head. War is almost ALWAYS worth it for humans because the AI is really not good at tactical combat. I have yet to take a stab at Deity in BNW but in G&K I considered going early war with an early war race (Monty, Genghis, Atilla) akin to cheating because it really wasn't hard to "win".
The only BNW game I've gone conquest on was an Emperor-Zulu and it was ridiculously easy. I wound up with -30 happiness, but this turned into a boon as the endless rebels showing up in my territory provided a constant flow of cash and culture for my Honor-bound Spearmen and then Impis. It actually felt somewhat exploitative.
You have to be comfortable running at a major deficit (you will make gold through murder), puppeting everything in sight (even cities you want, you can always annex them later) and be good at dealing with massive unhappiness (your puppets love to spam happiness buildings to help you out here). The result is an easy victory at just about any difficulty level (I can't speak for a warmongering Immortal or Deity in BNW yet, but I will come around to it).
It really REALLY is the easiest way to turn into a run-away unstoppable juggernaut. In my Zulu game I had planted my flag in 4 capital cities and had completely wiped out 2 civs by turn 150 and had 3x the score of 2nd place, who was the next "target". Oh yeah and with just about every wonder in the game (conquered Ramesses) at that time my economy was bustling from all the side bonuses.
The catch of it is that it's almost the polar opposite of playing the traditional "builder". I personally am an instinctual builder so I really have to put myself in the seek and destroy mindset to go warmonger but once you figure it out it's just a different skill with different goals (never strop breaking things) and by the time you HAVE to slow down (start losing cities to unrest) it's too late for the AI. Then you can take a hard earned breather and either go hardcore wide-empire builder mode and win however you like OR just upgrade some units and fire up the war machine again.