It seems, that stability issues are indeed directly related to the version/assembly of u-boot. I've built complete image of Ubuntu 14.04.03 LTS on 11.11.2015 which identifies itself as:
Title: Armbian 4.6 Cubietruck Ubuntu trusty default (unofficial) Kernel: Linux 3.4.110
It was quite unstable and could be easily made to hang completely by running "memtester 1500'. Then u-boot was downgraded to 4.5 via apt-get while keeping kernel the same. After that, the board become stable and survived several days under the load of memtester.
There seems to be an additional issue of stability with 2.5" hdd connected, but it manifests itself even with original Cubian image. Preliminary fix for it includes soldering of 220uF electrolytic capacitor directly to the pins of DC-IN jack. In both cases, board was powered via "official" 12to5V DC-DC converter.