Personally I believe that some (not all) bugs don't appear in their RC. For some reason they only appear when the update goes live. So no amount of testing would've helped. I can live with that as long as there are efforts to fix these bugs ASAP.You test, look at feedback on performance (i.e. what did people find wrong / not working) and once the major problems and listed fixes seem to be fixed in follow up feedback / build then you release. You don't release yet more untested code because you are then undoing your testing phase and making it redundant. The stutter is an excellent example of this- it would have been instantly commented on, but was not because it was a new problem introduced in a new build. I found severe (i.e. CZ breaking) bugs in my testing, I can't know until live if they sorted them out.
The official feedback area, i.e here: https://forums.frontier.co.uk/forums/january-update-beta.409/
On the other hand, I believe there are also bugs introduced by changes they made after the public beta which could've been identified by QA if they had more people / time / experience.
So in my opinion they should always have a public beta for the RC and only go live if no more issues* are reported. But in that case we should also accept that bugs are still possible.
*no more issues = no more general bugs which happen on every computer and are easily reproducible. I understand they can't fix every issue for every configuration.