It goes both ways.
No you really can’t understand a culture from a week of study the same way you can from being there for a week. The issue is the millions of unknown unknowns that you never really consider. How large is people’s personal space, where do they stand and look in an elevator, what’s traffic like, how loud are people, etc etc. Of course a week or three isn’t that long, but there are real diminishing returns here.
On the other hand personal experience is very narrow in scope. You’re never going to find out country wide crime rates by wondering around for a week.