... if not even more rude to comment on what people have left on their plates as it is to leave food that somebody has cooked for you?
I am not a picky eater, and will have a good go at anything really, but there are two places where I draw the line- creamy sauces eg carbonara (the smell of the cream makes me feel really, imminently sick) and beef (I will have a stab at it but I find the texture horrible. This may be because of my HFA and therefore unreasonable.)
If the meal is a "surprise" eg the host hasn't told you what they are cooking in advance giving you a reasonable period to back out if you really can't stomach the food, and it's a situation where they'll be offended if you say you'd rather not have any of x thing (because they might cook the best carbonara in the world, but you just don't like carbonara!) then AIBU to think that while some may think it's rude to cut up a meal and "pretend" to eat it so as not to offend