A few points about why some peptide are of more concern than others
-The closer a peptide is to a naturally occurring protein present in the body, the greater the risk from immunogenicity. It's one thing for MT-2 to become less effective because your immune system learns to treat it as an enemy invader, it's another for you to develop antibodies to your own growth hormone or GLP potentially weakening their effectiveness for years, or forever.
-Risk is increased with exposure. Few peptides are injected daily, year after year, like rHGH. This is becoming common as the price drops.
-Most peptides aren't folded or have other "complications". They're simple strings of amino acids. In those cases purity tells you all you need to know. When there's are complication, like a folded structure that's important in the case of rHGH, or in the case of GLPs, a special lipid tail to extend its half life, these things are susceptible to going wrong and undetected by normal testing. An incorrectly manufactured tail on Tirz not only shortens half life, it can cause it to not be filtered out of your system. The correct tail attaches Tirz to albumin, for a predictable half life and clearance via the kidneys. I know of at least one example of UGL Tirz with an incorrect tail, which not only shortens how long a dose works, it can cause it to accumulate in another organ instead of being cleared, with unknown risks as it builds up over time.