I've only had mine a little over a month and, knock on wood, I have not experienced any issues. I am running Merlin 3004.388.4 on it.
I went from an AC88U to the AX88U Pro, and four vs two cores might have helped a bit. Having said that, any increase in download / upload speed is, I feel, more to do with the fact I'm able to use a media convertor that syncs at 2.5G which is plugged into the 2.5G WAN port, so I get the full 1.5 down from my connection, which prior to that was 1G due to the 1G port on the AC88U. It's not a massive difference, but every little bit helps.