Its true, its much easier to fix than the stuff under the hood. It was so simple, Nokia screwed it up twice in 3 years even with a HUGE r&d budget!
The UI overhaul on n97, horrid.. horrid experience
The UI overhaul on n8, more consistent but still feels like early 2000.

There's absolutely no reason to expect them to be successful a 3rd time round. The problems are probably due to
- 10+ year old Symbian code turned to spaghetti, code changes very difficult to make without a domino effect over the rest of the code
- poor poor project management and 9am-5pm paycheck Symbian developers who are there just to fix bugs.. not rock the boat

i get the impression the Symbian software department was so fundamentally broken it could not be fixed without mass firings