I just recently griped about this in another thread but I dislike how their stupid calorie system pays no attention to any "recommended calories per day" thresholds and instead just adds it all up so that you'll inevitably gain weight if you just eat. As someone who plays with long life span that caused basically all my sim children to get fat because, surprise, they kinda need to eat while having very limited exercise options.
There are some sims whose physique matters to me because of roleplay/immersion but the system made it so that if I wanted to keep their appearance at certain level then they all needed to specifically hit the gym or go jogging which then earns them fitness skill. That's not something I want because why should everyone be skilled in fitness? There are skinny people irl who basically never do fitness and there are larger ones who regularly do (and have better endurance/strength). Thus for certain sims I'll just unashamedly use MCCC to either freeze their physique or change it on the fly.
Like, I was playing my Homeless challenge and it got ridiculous how they repeatedly got fat even though they were basically starving all the time and then bought some apples or curry or whatever from vendors, definitely not just feasting on greasy fast food. It just broke my immersion because the game ignores all the other elements of their tough lifestyle. Sure, they might not hit the gym but standing for 5 hours fishing ain't actually slacking around either is it?