We have lots of unit tests, and also a test suite which launches a couple of local lemmy instances and ensures that they federate as expected. But it’s not possible to cover every single functionality, at least not with our limited resources. The problems all happened with things that are difficult to test and had major breaking changes in this release. In the future we won’t need such breaking changes so there will be less problems.
Also keep in mind that Lemmy is provided for free and as is. We have no legal obligation to users. And you can always stay with an older version if you want more stability.