Well, there are more out there than people think...maybe not a ton, but more than is obvious.
Once I got pretty good at 1c Plate 1L colors and their progressions over time, years ago, I started noticing #23's with early colors (1852). That was a major red flag. I don't know if its enough to call a given stamp out as bad, but its strong circumstantial evidence to me -- and I saw enough of these, to where "they are out there".
Overall, I mostly agree, that it hasn't seemed to be a huge problem off-hand.