I have been using the Xstream, since it was first released in the early 2000s, along with other upstream and downstream Poseidon models, dating back to the 1970s; have never had a single issue with any of them; and have only witnessed one over-pressure failure, while working dive boats, years ago, with a poorly-maintained Odin (the Jetstream, which has, basically, the same interior components as the Xstream second stage).
The diver was certainly frustrated, but experienced no difficulty in his breathing, while the OPV valve (then on the hose) vented excess gas, during his controlled ascent from about twenty meters -- much to the surprise of those unfamiliar with how those upstream models functioned, who largely believed, back then and some now, that they would somehow "fail-closed" and cut-off an air-supply.
Not so . . .