Depends on how they're built, and building them takes experience. Plus, if their MTBF is long enough with enough hot spares, you can rotate the problematic ones out fix them while they are being replaced from the hot-spares pool.
When you are not budget constrained, and building things for businesses, a little overengineering goes a long way.
I have a Xerox 7500DN color laser printer next to me, and it's working for more than 20 years at this point. It has gone through a lot of spares, but most (if not all) issues are from parts wearing down naturally. Nothing breaks unexpectedly on that. Same for robots. Give enough design budget, overengineer a little, and that thing will be one hell of an ugly but reliable machinery.
When you work with real "industrial" stuff, the landscape is very different.
When you are not budget constrained, and building things for businesses, a little overengineering goes a long way.
I have a Xerox 7500DN color laser printer next to me, and it's working for more than 20 years at this point. It has gone through a lot of spares, but most (if not all) issues are from parts wearing down naturally. Nothing breaks unexpectedly on that. Same for robots. Give enough design budget, overengineer a little, and that thing will be one hell of an ugly but reliable machinery.
When you work with real "industrial" stuff, the landscape is very different.