Note that even an open beta won't necessarily catch all the bugs.
For instance, for CK2 the Way of Life DLC had an open beta for the main patch (not the DLC features). It caught some bugs, but one of the fixes led to the notorious "levy bug" (the AI was unable to disband its levies; given that levies must be disbanded before declaring war in CK2 and vassals get angry when levies aren't disbanded in a reasonable time, this meant that the AI was unable to declare war and all its vassals became incredibly discontent within a few years of the game starting).
For the forum's reaction?
This video is a fairly accurate summary.