It's not that I can't sustain it all the time, it's just that I don't think about it on weekends for the most part, so if I'm going out then I won't go out of my way to stick to a specific schedule. So while I am still fasting, it may not be strictly 12 or 18 hours, for example.
My main point is that you can lose weight by adjusting your habits and what you eat without thinking about calories at all, but I am biased towards a meat/egg-based diet based on my experience with it.
Insulin resistance and metabolic dysfunction are what I would consider the driving factors of obesity; if you suffer from those issues, then carbs are going to hamper progress if the goal is weight loss, but if you don't, then you are more likely to get away with eating from most food groups without getting fat. So in that sense, goals definitely play a factor.