I finished my ATPL theory exams early summer, so I know very well how this works. Bear in mind that I did the ATPLs before they introduced these new questions with lists, typed answers, etc, but at this time I have to say that the Bristol Ground School question bank was absolutely bang-on. AGK, Comms, Air Law, and General Navigation were (a few months ago) 90% from BGS online. The rest of the exams were slightly less, but all had a considerable amount of questions that I'd seen before on the question bank. Personally, I think I could have scraped over the line without using a question bank, but it wouldn't have been pretty, and it would have taken considerably more time and effort. That being said, some people absolutely hammer the question banks, and still scrape over the line, or even fail.
At the beginning of my course, there were a few people who made a point of "I'm not going to use question banks, I'm going to read the textbooks inside-out and know the content...". As honourable as their intentions were, they were the lowest scorers in the first set of exams, and they soon realised that question banks were they way to go if you want a decent average. A few sly remarks were thrown around between individuals at ground school, such as "You only got such a high average because you did the Bristol question bank from day one..." or "I got a 10% lower average than you, but I actually know the content, you just know the question bank...". I just feel you should do what ever is necessary to get the highest average possible, and give yourself a bigger feather in your cap. There's nothing dishonourable about it, it was the way it worked.
In my experience, the relevant stuff comes in when you do your practical flight training. There were some truly ridiculous questions that were more pub quiz questions than ATPL theory. Particularly in HPL, Air Law, and Radio Navigation. There's no relevance in knowing what a "first group" is, when the Chicago Convention was signed, or what types of atomic clock a NavStar GPS has. Easy marks, I admit, but completely irrelevant.