If shooting lots of rounds through them is your goal, they are not a good choice. Back in the 80's several LE agencies and even some smaller militaries tried them as a cheaper alternative to AR's. When used in that role they did not hold up well and were quickly dropped.
Urban legend. Not so. What is so is that the Mini platform operating system is not suitable for being adapted to full auto due to heat considerations. But round for round on semi-auto I have never seen any test case that showed that a Mini had a greater mean time to failure than an AR. Quite the contrary, ARs are notorious for needing to be run "wet" with a lot of oil on the bolt and a lot of cleaning. of the bolt carrier group. Cops don't fire their rifles all that much anyway.