Stairs have always been reliable to me. Knees, not so much. My last project in Vietnam was spent in the north - two cities roughly sixty klicks from Hanoi. We were doing pilot testing work for the replacement of their respective water treatment plants. For six weeks I didn't have access to a gym, so I resorted to running, and stair climbing. I'm not sure which one did my knees in, probably the stairs more than the running, but by the time I was ready to go "home" to Saigon, I could barely walk. A few days of rest and rehab on the stationary bike and I was feeling better again. Those bikes were interesting - you got a video display, and one of the viewing options was recorded rides through different locations. There were two 30 minute rides through San Francisco, one along the wharf and the other along the Skyline, which I've been down countless times. Which, is an odd feeling in a foreign country seven thousand miles away.
Commercial cardio equipment does break down at the gym, but it's also used and abused 24/7. That said, the only reason I would buy one for the home is if I lived in a remote area with less than acceptable outdoor weather a good portion of the year.