Many of the detectors that use a 9V battery backup have a poorly designed chamber holding the battery. Often the battery doesn't make good contact with the metal contacts inside the detector. This results in constant false positives that the battery is low. When it actually does run low, I'm forced to start this cycle over and over again of: find "new" battery, alarm still chirping, is the "new" battery really low or is it the battery chamber misaligned again, etc.
The dog loses another 3 months of her life from stress whenever this happens. I've never seen an animal so scared of a constant chirp.
So when these things start chirping at 2am, and I can't tell if the batteries in the drawer are brand new, instead of dealing with a scared dog, and possibly buying yet more new batteries, the detector comes off the ceiling and ends up in a drawer of blankets to dampen the noise as the chirping takes awhile to die after the detector is removed from power and battery removed.
Additionally, I help with many things around our condo association. I'm constantly changing batteries in these detectors. But first I need to find them. You hear a chirp that the battery is low, and now you have to find it somewhere. Maybe it's in the common hallways of 3 floors of units, or maybe it's a unit owner's. This involves me walking around the building trying to catch one in action of chirping.
I've been a loud detractor of Nest in the past because of their use of Facebook ads spamming my friend list, but I welcome an intelligently designed detector and am going to gladly spend the money to buy at least two of these and will hopefully get the condo association on these. The dog is going to be very happy. :)