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.
I used to work at a zoo. When you introduce a new animal to an exhibit, you soap the window so they don't hurt themselves in their confusion. Just... Take a bar of soap and fog up the window with it.
window binds still let the light into the house where the energy then turns to heat.
Heat-reducing window-tint on the other hand is pretty easy to install.
honestly, on the outside, the yoghurt probably isn't a particularly durable layer (i.e. going away after the first rain,) and probably gets to smelling funky. On the inside, it's probably going to also smell funky.
Maybe my understanding of blinds is wrong, but the ones on my window get pretty close to completely dark. Add a decent curtain (with insulation and blackout) on the inside, and I can keep the flat comfortable passively.
Whatever else you do, close the damn windows and doors during the day, and fling them wide open as soon as its cooler.