Let's say you want to study long term impacts of taking Viagra which is clearly going to take some time. Sure, you could look at old data, but that always has issues. So, you get funding, start some research, and want to publish something when your done.
Option A: Pick just one thing to look at say cardiovascular health.
Option B: Collect a lot of data, and then look for trends.
Option B has much stronger risks of false positives simply because your looking at more factors, but those false positives seem much more interesting which helps you get published. Worse you can't use any of that data for verification, so you now need to run another 10 year study. Upside, you got published, downside, your results are almost meaningless. Bigger upside, you then get to publish again in 10 years.
Well what about option C use approach B but publish sooner. Now your not only risking false positives, but also extrapolating from more limited data.
Hmm, guess what say nutritionists are going to pick... "Chocolate good/bad for you!"