As I understand it, the major problems with the old forums were back-end, such as security and handling user volume.
Correct, as far as what has been told to the community is concerned.
The problem (in my opinion) is that when they upgraded, they chose an interface and system so radically different that what people were used to, there was a huge backlash... and forcing the switch just aggravated it.
In my opinion, this wasn't even the main issue. They chose an interface and system radically different, yes, but it's also not even close to being structurally complete yet. It's very much still in beta, and forcing the community to use a platform with certain things that basically don't even work (lookin' at you, mobile support and search functionality) caused a huge wave of negativity.
There's also the fact that the Boards team put out a few threads that were perceived by the community as stifling or completely incorrect at the onset of the transition and has had no high level conversation at all since then. As far as perception is concerned, they basically forced transition onto a beta platform then completely disconnected from the community.
It wasn't just forumers that were annoyed, by the way; native Boards users were, to my knowledge, not too happy about the transition either. It just wasn't ready, and it still isn't.