They're actually certified for in-wall use too. That's the beauty of them, they are 100% reusable. Open and close them as many times as you want. I've basically abandoned wire nuts. They even sell a version that is 2x2 to splice a hot and neutral to another hot and neutral in just one connector.
I had one 4 gang switch box where the original electrician had piggy backed one to the next. So, take out one switch was a PITA. So, I took one of the 5 port connectors and 4 pigtails of wire and had it all neatened out in about 10 mins. Connecting 5 romex wires with a wire nut is impossible!