I have received mile compensation and $$ refund for purchased preferred seating loss, when there were cancelled flights, delays, re bookings, late arrivals because of mechanicals. But only after I wrote a short, concise, polite explanation email to CS.
When our DFW-HNL was delayed 3+ hours a couple years ago, due to a mechanical and eventual aircraft swap, the next morning we received unsolicited emails from AA with apology and advising that 5K miles would be deposited in our accounts.