A few words from a general point of view:
Mechanics experience minimal wear as long as there is grease, or a lubrication film. If bearings and gears are not sealed, then the air needs to be dry and clean. Perhaps as a measure, after spending a day in the factory, if it is black what comes out when blowing your nose, then the mechanics may wear out fast.
But I think that an experienced person would also point out that the quality of the individual machine component is important. Bad material quality, hardening and tolerances of rolling elements and bearing rings are negative for life length calculations, e.g. the L10 value (ISO 281).
One way to see the effect of bearing quality is to compare the effect on the life rating by changing the bearing quality in the formula (or lubrication contamination). For SKF, it would be to compare standard SKF with SKF Explorer. If you search on the Internet for ‘skf bearing catalogue pdf’ you can read examples and make your own calculations. You can also search for ‘Bearing rating life’.
On page 88, Table 1, you find that the design and operational environment have an effect on the order of magnitude 3 or 4. With low quality bearings, you will be in the top of the table, and with low load on the robot arm, you will be in the bottom of the table.
Page 88, Table 1 (SKF 2018):
Some less important notes:
For circuit boards, if they are kept clean and are OEM, they are reliable. Electronics reliability follows an exponential distribution. To make it more reliable, you add redundancy. This is important in safety critical systems, like elevators and airplanes. Then software reliability is critical as well.
Regarding the reliability of stepper motors, I think you can read on 3D printer forums, like reddit.
Also, you have some limit switches that you may want to check the datasheets for reliability figures.
