If your vial of cheap chinese Tirz contains the correct drug, then there is essentially zero difference.
I have no doubt that the manufacturer formulation is superior in terms of long term peptide stabilty, which it needs to be as it has to have a long shelf life after being mixed with water which is how they sell it. But given that the lyophilised peptides are only mixed with bac water as needed, and used in the next month or so, it really does not matter if the mixture is as stable over months to years after being mixed.
Apart from that, dosage accuracy is not as precise due to under or overfilling but the worst I have seen is 20%, and most are within plus or minus 10%. In practical terms this really makes no difference either unless you are especially sensitive to adverse effects from tiny dose changes.
The biggest risks are of being ripped off by a fake seller or one who gets busted, but even this is usually cheaper than paying for the original version. A loss of a few hundred to a bad seller is still way less than the cost of a one month prescription.
It is not impossible to get a product that has nothing in it or worse has a different peptide or toxic contaminants, it happens but is not common. The only way to be sure this is not going to happen is to get your batch independently tested, before you use it, either yourself or through a group.
Bacteria or endotoxin are possible as well, I have still never come across a case of someone complaining about infections at injection sites, so presumably this is quite rare, and tiny amounts of endotoxin are not likely to be any worse than the amounts that leak through your gut into your blood every day.