My DD has an Angelcare, she also had a hospital monitor for SIDS for the first ten months. The hospital one false alarmed all the time (I was a nervous wreck) the Angelcare only did it when the set up wasn't exactly right for the situation.
I wouldn't say it happens a lot, but it does happen - you can get false alarms with ALL movement monitors. But they can usually be sorted out by checking certain things and making adjustments.
First thing to check - do you have a full length piece of solid board under the sensor pad? Placing the pad on standard cot slats reduces the effectiveness and results in false alarms. We have a piece of plywood cut to fit the base of DD's cot, then the pad, then the mattress.
Second thing to check - is the pad slap bang in the centre of the cot? We found even the slightest movement in one direction caused false alarms.
Third thing - if your baby is able to move to the side of the cot it's worth adjusting the sensitivity setting. Once DD was mobile we had more false alarms until we turned the sensitivity up. We have a cot bed and she lies widthways across the very top. On standard sensitivity settings it didn't pick her up.
There is a lot of info on trouble shooting false alarms on the Philips website too.
It is so scary the first time you get a false alarm, but try to remember the vast majority of alarms ARE false, and now you know that you will hear it and respond should the need arise (I got to be very good at leaping out of bed and getting to the cot upright at the first beep! I have NEVER moved that fast!)
As children grow the settings on the monitor will need adjusting the more mobile they get.