Across all my customers, I probably RMA a failed device every 2 to 4 weeks (especially MX65s at the moment; they are starting to fail at much higher rates, especially units with both POE ports in use).
I have had a lot of MX65s replaced with MX68s.
I have had two MX400s replaced with MX250s (along with all optics)
I can't remember ever having an MX84 fail.
Note that the failure has to happen while still in support to get hardware replacement. Otherwise, the replacement is on you.