Overall I really do think that pre-prepped and convenience foods are much better than they were.
I'm sure they are better but they're usually still full of chemicals, i.e. additives, colourings, shelf life extenders, salt/sugars, etc.
The fact is that we just don't know enough about how our bodies cope with chemicals being pumped into them.
You have to remember that at one time (not that long ago), calories were calculated based upon how hot that particular food burnt - i.e. the assumption was that all energy acted the same way in your body. It's very recent that the scientists/biologists have realised that different types of food behave differently in the body, so 2 different foods with exactly the same calories can affect your body in different ways (i.e. fat storage and weight).
Since I've been diagnosed with diabetes, I'm a lot more aware of my food intake and how different kinds of food affect my blood sugar levels. I've learned to avoid all processed, tinned, prepared foods - even though some are claimed to be low sugar, they nearly all push my blood sugars high. Whereas, buying fresh/unprocessed ingredients (even the same kinds of foods in the same quantities), don't raise my blood sugar levels anywhere near as high. In fact, my blood sugar levels don't go as high when I deliberately eat high-sugar content foods if they are natural/unprocessed, and fresh. The chemicals added definitely affect my blood sugars. When they have that effect on sugars, they'll undoubtedly be affecting other elements of your body too.
At the end of the day, pumping chemicals into your body just isn't natural. I now avoid anything with an unnatural shelf life - so tins are out, as are packets (biscuits, crackers, corn flakes, cakes etc), and also wrapped bread is also out. Most decent sized supermarkets have a bakery, so I try to buy fresh bread, fresh cakes, scones, etc., all of which should be "bad" for a diabetic, but I find they don't raise my blood sugar levels as high as packets/factory produced stuff which will have preservatives in for longer shelf life. I also use a lot of fresh fruit & veg, and fish/meat from butchers (or a supermarket if they sell fresh meat). I really try to get foods which have been processed as little as possible which basically means avoiding anything that's been processed/packed in a factory.