I decide if something is worth it based on how hard it was to make; an infinite closed-circuit power system SOUNDS op (and in the case of solar panels it is) but if it was made from an SS soul shard spawning zombies, which die in quicksand (prevents xp lag) so that I can centrifuge the flesh, get methane, use a liquid transposer to turn the methane into a liquid and send it into a tank, then use said methane to power the whole setup, I consider that balanced because of the amount of time it took me to make.
When I was worse at the game I would use the water-bucket-in-watermill trick or maybe the turtle-cutting-down-Redwood doohicky, i'm better at the game now and I consider than not difficult enough to warrant free energy, when I get better at the game something like the methane setup will seem simple to me, so I might make a system using a chicken farm and some contraption to harvest the unholy noise that they make and turn it into sweet, sweet electricity.
After all, isn't the point of FTB to automate as much as you want of your world?
To me, there is a huge difference between having a block that created 512 eu/t *cough*ultimatehybridsolarpanel*cough* and an awesome, but bulky, system that I made myself which took a ton of work, so that at the end, I can look at my full MFSU and think "I earned it".