I'm not sure I understand your question as far as what the risk is to a blow back system. Blow back refers to a type of system some weapons use to cycle the rounds. Some are direct blow back, some use a delayed blow back to slow down the cycling system, there's another one I'm forgeting, but I cant remember what they call it.
There is no Risk involved, just different systems. Some are more complicated than others. However all of them work as intended for the most part.
For example most semi automatic pistols are direct blowback. I think the MP5(and others) uses a delayed blowback and the AR platforms use a gas system or a piston sysyem now a days.
Sorry, I didn't give more information on it. It's the type of blowback that the PS90 operates on which is similar in operation to pistols. There are better designs out there for weapons with higher pressures that reduce the risks associated with higher pressure rounds.
There have been a few PS90s blow up due to slight over pressures that would not have occurred in other bolt designs. The KBs happen not due to the chamber letting go, but the casing exploding when being ejected from the chamber before the bullet has left the barrel.
The post that is in question was one of which someone was wanting to know about modifications which looked like it was supposed to turn a P90 into a battle rifle, which it's not designed to be.
There is a lot of information on the various blowback designs in firearms that will turn up in google. The P90 / PS90 is a direct blowback design which will operate if the barrel is allowed to "float" or not.