My neighborhood gets a lot of break-ins, and I've tried to understand whether there is a causal link showing that electronic home security lowers break-in risk. The deterrent effect makes intuitive sense, but it's surprisingly hard to find empirical evidence for (even recognizing that it's not the kind of thing you can run an A/B test for). Additionally the internet is so awash in marketing (there's almost never an incentive to tell someone not to buy something) that it's very hard to ascertain the facts, and consumers' behavior in these matters tends to be guided more by perception than true solutions.

I had a similar experience when considering a water filter. In that situation, I was able to actually figure out a sensible path, thanks to a very few unbiased websites: test your water or read your locality's water reports to figure out what your drinking water contaminants actually are, then figure out specifically which filter (if any) will address those specific contaminants. Meanwhile, plenty of people will buy a mass-market water filter for "peace of mind" which may actually be neither necessary nor sufficient.