They're pretty good
I spent a year developing a Stratus hosted system back when they were 68xxx boxes. I only remember it going down twice. Both times that was because the computer room power supply was tripped out during a weekend. The Stratus backup batteries were flattened after several hours without mains power.
There was a Tandem NonStop machine in the same computer room and ironically, both the outages were caused by a major failure in its PSU.