The primary motivation behind the ABCD categories was politics . . . the need to set up a European set of standards so Italy, for example, couldn't refuse to import boats from France, for example, because they didn't meet some "safety" or "construction" standard.
As we've seen over the years, boats of all shapes and sizes make successful ocean passages, sometimes after they've been abandoned by their sailors.
Since the EU standards came into affect (about 2000 if I recall correctly) just about every sailboat built that's over 30 feet has a Category A certification. US builders use the same standards so they can export to the EU.
CAT A by itself says nothing about how well the boat is put together, although other parts of the standards do cover safety aspects like downflooding angles, robustness of hatches, etc.
And, many boats built prior to the EY standards are perfectly seaworthy, too.