I used it to seal around the edge of windows that were leaking. It worked well for that but so would have regular silicon caulk. The flex seal was on a clearance table and cheaper or I would have just gotten caulk.
My take is that Flex Seal is for people that don’t own a caulking gun and don’t need to do a whole lot
I use it on our sun room. Much easier to apply than caulk when you are standing on a glass sun room. I caulk it every 3 or 4 years. In between that, I hit some of the joints with flex seal, and I have never had problems.