I can report that I had decided to 'upgrade' from my 68U running Merlin (rock solid BTW) to an AC-86U running the same firmware version. All was well for two days, then the dreaded wireless drop outs which then progressed to many, many reboots. After a week of digging into this issue ( researching, modifying settings, resetting, restoring, reverting firmware to earlier versions, wash, repeat, then frustration) I have given up. I have had a dozen ASUS routers running Merlin over the years and I can clearly state that there is indeed something amiss with this router.
My RMA is in, but I am skeptical of the reliability.
As far as debugging, it was frustrating not to be able to zero in on a set of circumstances that trigger the Linux reboots. If you can reliably cause the problem then you have a chance. But this issue is elusive. The Loch Ness of Merlin if you will.
SO that is my experience, and it left me frustrated and less comfortable with trusting ASUS routers going forward.
Regards
Ed