I’d consider using something like this if I had already deployed marketing capital elsewhere and hit diminishing returns. These are more akin to gambling in my book.
Not to say that polygon-based geofencing doesn’t work. It does, but with larger, less dense locations where the lower fidelity is good enough.
I'm not so sure in this specific case. If I were a (cue pejorative stereotype for emphasis) scumbag ambulance chasing lawyer - I doubt there's anything in Facebook or Google's ad targeting demographics that'd give me quite the same opportunities as a rough-and-ready demographic of "people at or close to hospital emergency departments" to try selling to...
I hope (but am prepared to be shown otherwise) than neither Facebook nor Google have an easy way to target ads to people who've just posted "Faaark! Just been in a car accident! #YOLO #GladIBoughtAVolvo #ShouldntHaveBeenTexting"
(Evil-idea: I wonder if anyone's using siome combinations of accelerometer and GPS data to detect potentially traumatic events and selling that data on?)
(I'm equal parts horrified and impressed that someone's actually doing this... This is why, as much as it's annoying at work, I really like the GDPR...)