I've never used the bubblewrap so will let someone else speak to that issue, but my greenhouse (a Rion, with twin-wall poly), if not heated, will also be essentially the same temp as outside at night; once it's light out, though, it warms up much more quickly (even on a very cloudy/rainy day). And when the temperature drops outside it takes somewhat longer for the temp in the GH to drop. Although I never used the mylar blankets, I don't actually think they will keep the plants warmer at night, but possibly would provide frost protection; I usually use some 'floating row cover' if I'm worried the temperature is going too low. Or, if your plants are on (or planted in) the ground, covering them with the blankets would probably hold in some of the ground heat.
