If you're talking about a good CO switch like a 5ESS or DMS-100, they're extremely reliable, well engineered machines. They consistently meet and exceed five nines reliability.
I even ran a 5 nines 9-1-1 service off Asterisk, as embarrassing as that is. We missed one call in 18 months during a hard maintenance cutover, but handled it manually, but of course it was only one part and relied on the rest of the PSTN.
With a SIP proxy in an HA setup, almost surely the biggest downtime is going to be operator error or IP connectivity issues. Or someone changing their signalling subtly, but OTOH 5ESS and DMS aren't totally identical in behavior either.
(Though fair enough, the "software" guys haven't shown the same uptimes, overall, as the old style switching, true.)