Your thyristor circuit sets your over voltage trip point. You can analyse and test this circuit and I would expect the over voltage trip point to be completely stable. Be careful with alternative parts though, as the thyristor gate voltages can vary significantly between otherwise similar devices.

Once the crowbar has tripped it will draw a current through the polyswitch fuse until the polyswitch fuse reaches its current trip. The polyswitch current trip may vary significantly (just like real fuses), but that doesn't compromise the overvoltage protection. It does mena that the thyristor needs enough current margin.