Oh FM8 is powerful as hell, absolutely. My issue with it is that when it comes to sculpting a sound, it's very bipolar. You either work with the presets and get minor control over tone and timbre with an "easy" version of the synth controls (which can get you a long way, truth be told), or you dive into the deep end with complex routing and ridiculously in-depth tweaking. It sometimes feels like choosing between IKEA, or cutting down a tree and sanding it down to a chair every time you need to sit down :)

Operator is deceptively powerful, especially for bass sounds. You can get some terrific tone out of it and I find fudging around with it can be a bit more intuitive and straightforward. Also it uses substantially less processing power, which can be a concern when you layer that shit up as much as I do. 10 operators versus 10 FM8s or Massives will be a lot lighter and easier to tweak in real time.

Having said, I find myself more often than not using both to an almost equal degree the more I learn about them and figure out what's computationally cheaper while maintaining the analog warmth.

OR these are just excuses for me not knowing my pears from my apples and RTFMing FM8 :) but I prefer the more glibby excuse.