Hi Jakob,
I'm really sorry for all of this.
Actually, I can work well with SPPB 4 personally. There should be no major issues if all of the requirements are met and the environment is properly set up.
Yes, there are many forum posts you will find reporting issues, but most of them are about conflicts with the environment and requirements. SPPB doesn't contain anything that can break the site. It can only cause some style changes that seem to appear differently after the update. Because of bootstrap, font awesome, and other version changes, they added some default styling to force container width, class, and so on to certain elements, among other things.
We have some internal settings-related issues that our developers fix after getting reports from users. Maybe Paul put some extra caution while advising so. Because it is always true that you can adapt to a new thing after it gets old enough or mature enough. But it doesn't mean the product is unusable. As I can personally use it at my end, it behaves differently under some special circumstances.
We all have different points of view. Thanks for being with us. Have a nice day!
Best Regards