How would we know?
If we know about bugs, we fix them. But, in a major version, there will be a period where there are bugs that we didn’t find in testing and that haven’t been reported yet.
However, there seems to be some confusion about where issues occur and what “upgrading to Virtualmin 8” means. Installing the Virtualmin virtual-server module verson 8.0.x is like any other virtual-server module update. You should generally upgrade when it’s released. That’s never expected to cause serious trouble.
Where there have been bugs and issues that needed attention, and where we have been advising caution, is in switching to the Virtualmin 8 repos. That is a dramatic change. Dependencies change, available packages changed, Ilia maybe jumped the gun on making a Webmin minimal package which caused quite a few issues for folks who rushed into switching repos.
Nobody has told folks who installed during the Virtualmin 7 cycle to switch to the Virtualmin 8 repos, in general. There is no rush to do that. You don’t get new functionality by doing that. The new modules will continue to be released via both the 7 and 8 repos. The installer changed how it configures things and what dependencies are installed, by default, but if you’re not running the installer on a fresh system, you’re not getting any of those differences by switching to the Virtualmin 8 repos.
In short: When a package goes into the repos you are on, you should generally upgrade when you have the time to do so. When a new installer and repo is announced, you are under no obligation to switch to it and existing installations don’t gain anything by doing so. You already have the new versions of the packages, and will continue to get those packages for the life of the repo (usually a couple of years past the release of the next version).
Think of the repos as like a major version of a Linux distro. We can break things during those changes (we’re not trying to break things, but packages go away, new packages and needs arise, etc.). So, if you don’t want breaking changes, don’t switch repos! Nobody is encouraging you to do so.