There is an extremely important detail left out of these conversations on motion sensor monitors: the care that must be taken when setting them up and adjusting the sensitivity setting knob on the back of the unit (for Angelcare monitors). Most people treat these as "plug-and-play", leaving them at the factory setting or setting them at the typical recommended setting of 3. In fact, establishing the precisely correct sensitivity setting for each individual setting is crucial. Mattress type, presence of ceiling fans, bedding, baby weight, baby age, etc. - all these require different sensitivity settings. Further, as the baby grows and as changes are made to bedding, the sensitivity must be recalibrated. Such recalibration requires an analytical mind, patience and experiment. Unfortunately, most people don't have the experience or skillset to do this. It's not their fault, it's not that they're dumb, it's just that not everyone thinks like a scientist or engineer. I have found the Angelcare motion monitor to be extremely reliable, accurate and trustworthy, and I have no doubt that it could save baby's lives, but only in rare cases where the sensor have been setup and regularly maintained, which I doubt many people do correctly. Pediatricians know this, too - so they don't want to be liable for a lawsuit or at least a guilt conscience when someone uses a motion sensor improperly. As a Ph.D. engineer and a father of a preemie, I would swear on a stack of Bibles that these things can work fantastically in the right hands, but that 90+% of the time they are probably not being used properly. If Angelcare engineers wrote the proper procedures down, it would be a 50 page manual and would leave most people - rightfully - feeling like it's too difficult. Bottom line: these devices will work perfectly if you're willing to sit down with the monitor a few hours a month to recalibrate it, while you baby is sleeping. That means in the dark, on the floor, with a flashlight, after your baby has entered a deep sleep cycle, and with all room conditions at a normal, state. As newborns gain weight, recalibrate. Old enough for a blanket or pillow - recalibrate. Ceiling fan clockwise for winter - recalibrate, counterclockwise for summer - recalibrate. A/C off and heat on for winter - recalibrate. Etc., etc., etc. recalibration is done by moving the sensor to it lowest sensitivity and then stepwise gradually increasing the sensitivity until actual breathing movement (combined with room air movement) is detected (no more warning beeps). Then you start at the other end with the highest sensitivity and gradually reduce the sensitivity until you lose the movement detection (the warning beep starts). All the while, of course, you have to keep the monitor from going into full alarm after the warning beep by turning it off, then on again. Eventually, you should end up at a sensitivity level that is somewhere between these two threshold levels. Then you have to sit and wait and watch. In the morning, lift the baby off the mattress and time how long it takes for the alarm to sound. If it's much more than 20 seconds, then your setting is wrong. If you do all this calibration work and still get apparent false alarms, then you have due cause to ask your pediatrician for an order to get a professional home heart and respiration monitor (with chest sensors) to gather more data and see what's really going on.
But all these baseless claims that these devices categorically don't work and can only give false-assurance are completely wrong, in my experienced opinion. Frankly, they're just not designed and fabricated at the level that they work perfectly out of the box, under a wide range of (constantly changing) home conditions, for your average person.