So I took the plunge and put the latest Merlin firmware on it. It worked fine -- rebooted and got to the GUI. Then I wanted to do a full reset -- nvram erase; nvram commit -- didn't seem to work, then I did a reset via the GUI. After rebooting, I had the same results as others -- incomplete GUI loading and eventually a full white screen for the GUI. Checking via curl showed there was some issue with the web pages it was serving -- I got
curl: (56) Recv failure: Connection reset by peer
at the end of the GUI.
However, since the router seemed to have booted, I tried to directly go to another config page -- I randomly choose one of the .ASP's pointed to in the HTML (that I got from curl), and tried
http://192.168.1.1/Advanced_ASUSDDNS_Content.asp. That got somewhere -- a GUI, although it was malformed and the language selection was overwriting many of the fields. I was able to change some settings, and when I hit 'apply', that page came back OK. I then was able to see the main menu on the left -- links from that gave GUI pages that weren't complete, but I was still able to fill in fields. As each page was 'apply'ed, that GUI worked. I rebooted and tried some other pages (including enabling/disabling features just to be able to 'apply' them).
Now it alls seems to work. One thing is although I used the Merlin AsusWRT for an RT68U, the GUI title page still says TM-AC1900 -- how does it 'know' this ? I erased all the NVRAM.