The number one concern with moving mechanical parts on today's smartphones is longevity, so the OEMs are always fast to assure that they've tested their products properly before releasing it to the public. The Mi Mix 3, for example, is promised to withstand at least 300,000 slides but an independent testing from a Chinese media says it might go even further.
Digital Tail uploaded a video on Weibo showing off a Xiaomi Mi Mix 3 smartphone attached to a machine that tested the mechanical slider going up and down 600,000 times - that's twice the amount Xiaomi promised during the announcement. It's quite impressive that the slider works perfectly even after the test finished.
So if the slider keeps you from going over the fence, don't worry - it probably won't fail you in the next 7-8 years of usage if you are planning to keep the handset for that long.
But what about dust between the 2 sides? It will effect the phones slider defo!
So your going to take 300 selfies a day? That's really the only reason to slide the phone open.
Users won't wear out the slides since they will rent a new phone after two years.