IEC contacts pass the exact same NRTL life testing (nominal 10 million no-load and 1 million full load operations) as any NEMA device. They are smaller because their terminals operate at 75C versus the NEMA 55C, and they are not designed to be repaired.
They are often misapplied because they are available in so many different ampacities. But there is nothing inherently wrong with them.
For example: an Allen-Bradley NEMA size 1 contactor is rated as 27A continuous, 32A service limit, and 10HP@480V. Now you can chose an AB IEC contactor based on 10HP@480 and get a 16A device or chose a 27A device and get a 20HP contactor.
The selection choice is yours, but remember a 10HP 480V motor has a NEC current of 14A, and a typical running current less than that, which means when pushed to the typical service factor of 1.15 the motor will still only draw 16.1A max.
The adjustable overload is the same concept as any NEMA bi-metallic or even the new solid-state designs. People will misadjust them, but they will also put in incorrect heaters or even jumper across them.
