About as sturdy and reliable and any other one.
Youtuber Spiritplumber devised a cheap and relatively-simple way to extend your cell phone's battery life. Recording video on smart phones is notorious for draining your juice and he claims this workaround can last a few hours with just a couple of bucks. Or you can go the less ugly but more expensive route and just pick up one of these guys.
"I have a good cellphone with all these neat features, but I couldn't find a cheap bike mount for it, so I decided to build one of my own! I've just used cardboard, a lot of black extra strong gaffer tape, about ½ meter (~1' 8") of velcro and a piece of sponge that sits on the back, between the handlebar and the bike mount as a shock absorber."