Namely stuff done to the supplement at the point of production which makes it appear to be something it's not. For instance, with respect to protein supplementation, a number of protein powders a couple years back were accused of fudging the numbers by pumping up the nitrogen count using cheaper amino acids, so rather than getting strictly whey protein, you were getting whey protein cut with cheap non-essential amino acids like creatine, glutamine, cysteine, glycine, etc.

Then there's contaminants to worry about, as I remember reading something about heavy metals making it's way into whey from China or something.

Technically they can do similar things with cuts of meat, but I imagine that's more difficult to get away with unless it's processed meat.

That being said, I still buy the cheap bottom shelf whey protein from walmart to make up for dietary deficits when I'm too lazy to cook beans or something.

Not sure if we can even come up with a definition for 'better' , seems like we would have to run some long term study in an extremely controlled circumstances.