So... Yeah. In the earlier versions of Glock pistols, Gen 2, they discovered higher-powered calibers, even 40 S&W, were capable of causing frame failure. So, they started introducing a 'Third Pin", high into the rear of the Locking Block above the Trigger Pin to support the Block during recoil and spread-out the forces acting against the frame... Polymer 80 included this updated design, which Glock retained all the way through the end of Gen 4.
The GST-9 however... Two-Pin design. When asked about higher-power calibers, i.e. 40 S&W and 357 Sig, 80% Arms initially said (and I am paraphrasing of course),
'heck yeah, our awesome frame is so awesome it can awesomely handle anything!'
Then later, they seemed uncertain about 'mere' 9mm +P, and walked-back their previous statement of awesomeness, saying that the GST-9 '
stands for 9mm, and ONLY 9mm'...
Possibly just some miscommunications with their CS and technical staff? And to note, Glock's Gen 5 is back to "Two Pin" designs, either because their new polymer is strong enough to take the stress, or they just decided saving $1 on the third pin will be more economical than replacing a few cracked frames over the years?
Much more of a concern is the GST-9's rear rails, which are notably thinner than P80 rails, and it was discovered that in some cases, the slide has enough 'rocking' free-play that stable Sear-engagement becomes a concern. I don't remember hearing of any actual malfunctions due to this however, and not all slides seem to be loose enough to cause slide-rattle -or perhaps it is an inconsistency in Rail manufacturing? The way I test for Sear engagement would pretty well prove functional safety or not though, and I don't do too much all that special, just a few dynamic tests that aren't in the book.
Some Builders reported the Nickle-Boron coating wasn't that durable on the Rails or the FRLB component. I never saw any chipping or flaking personally however, and if it all chips off, then what? No one else had the coating anyway, so unless that made the rails much thinner...
No experience with the MOD-1, no idea what (if anything) they have done about the rails. I don't see much negative comment about them though.
The issues are, as you alluded to, more an issue with the company itself, that they lied to customers for several months about production and held on to "pre-order" sales money for many months longer than promised. They stole tag-lines regarding 'First Time Quality' from MGB after refusing his technical assistance or feedback, and allegedly subverted him on social-media as well. So, the denouncement of the GST-9 is overall a 'political' one. I personally have nothing against any Builder who wants to have the GST-9, and fairly little against the product itself thusfar, but have a lot of negative feelings about the company that makes them. But that is just my personal take.