Well for one, I never use the term "upgrade". Each site change is an "update". Because "upgrade" means different things to different stakeholders. Tech values stability and security. Business values ad performance and traffic. Community values UX and features.

The biggest issue we have is that there is essentially zero standardization, even frequently within the travel & leisure department. That's a result of acquiring forum sites for 20 years and attempting to manage expectations for 2 million users. So as we move towards standardized sites, the worst part is of course going to be the bumpy ride to get everything in line. Once that's all set, then we can make tweaks to the vbulletin core.

I also want to say this: Compare 6 or 7 years ago. We'd do a site update and it would take a month or more to iron out all the glaring bugs. But now (thanks to standardization and process changes), feature-removal notwithstanding, the changeovers are much, much smoother.