Reflectivity is good, hot spots are not. Just because mylar blankets and aluminum foil are more reflective than white walls or a grow tents mylar doesn't mean it's better for growing plants. Mylar will reflect a grow light and light burn your plants.
We've heard this a lot, too, but have never been able to see or measure a catastrophically high light intensity anywhere. While I'm sure it's theoretically possible for it to occur, the chances seem to be very slim as long as one is keeping the material somewhat flat and don't intentionally try to make it happen.
We've been able to create a hot spot with a homemade reflector once. It was a stainless steel bowl that had a polished inside. Although, this hot spot was very visible and impossible not to detect. According to the quantum sensor, the most intense part of the hotspot was around twice as bright (1000 PPFD) as the average light intensity just outside the hotspot (500 PPFD), and this was very much visible.
I've never heard of a case where someone has actually burned their plants with hotspots created from foil or emergency blankets. How much more intense than the average light intensity would these spots have to be? If plants burn at, say 1500 PPFD (just to throw a number out there), that would require the hot spot to be 5x more intense than the average light intensity in the wardrobe. If there was a (small?) hot spot that was 5x more intense than the average light intensity (of around 300 PPFD), it would be very visible on the floor, or if it was positioned right in a hole, it would be picked up by the sensor.
If one is growing at higher intensities, such as 1000 PPFD, it would still require the spot to be 50% more intense than the average light intensity.
I'm happy to experiment more with this if you have any pointers on how I best could achieve a hotspot with foil or blankets. If we can create a spot 50% more intense the average light intensity, it might be something to worry about.
By the way, we don't sell any of the materials tested in the video, or any other reflective material and have no own interest in what material is best. We simply just want to share information that helps the average grower get the most out of their lamp and plants.