FWIW, in my experience with DNN since v2, it's rare that you have to resort to rebuilding a site. In fact, I have never had to resort to that. I have found that it's typically the fault of something other than DNN itself.

I had a case when moving a portal from DNN v2.12 to 3.x that the upgrade went fine, but a previously installed/deinstalled module had left a roque dll in the bin folder that crashed the portal - I finally isolated it by doing a systematic uninstall of a test portal of after market modules. At the time, this had surprised quite a few veterans that a rogue dll could cause trouble, but easy to imagine if you think about it.

I had a serious error occur just recently when moving from v4.6 to v4.8, but after a few days of inspection, realized that it was a fault of mine for straying from the core registration by using an aftermarket module for user registrations, and not realizing the implicatons of my changes. The way it exhibited itself was through the core move towards true localization while I didn't pay any attention to the implications of not specifying a userprofile setting for preferred locale. It seemed trivial at the time.

When you think about it, DNN core can only realistically test against core upgrades.

And honestly, the last time I heard a compalint/resolution similar to yours (2 years ago), was from a user that would experiment with modules and then deinstall them - which they wouldn't do completely - and then be stuck because of rogue dl's.

I'm not suggesting that you are the cause of your own problem, but merely suggesting that there are so many other factors that might affect the success (or not) of an upgrade.