Well, disadvantage of only barely affecting temperatures though. 3.5C max, 0.6 average is not nothing, but for me at least, part of the problem is the fact that if I don't run AC, the temperatures just keep on climbing indoors. The 5-6C drop of tinfoil sounds more useful, but then they didn't really mention what the average drop is.

Granted, I realize most people would rather get light through their windows. But personally in bedrooms I'd rather take 0 light as otherwise you only really get 3-4 hours of dark per day in the summer. For other rooms - maybe some of those heat-reflective films? 3M claims theirs manages reduce heating by quite a lot, but probably not as much as foil.