I used my iPhone 11 Pro as a bike computer for a while with the Cyclemeter Elite app (I'm now using a Hammerhead Karoo 2). There are definitely a few drawbacks, but honestly, it's better than a bike computer in several ways. I had my phone mounted using a Quadlock case and mount. If you are going to use your phone on your bike, I highly recommend the Quadlock system - it's really strong and secure and well designed. Here are the pros and cons, IMO:
Pros:
• excellent screen - big and bright. No problems reading in the sun
• Cyclemeter app is fantastic, almost infinitely configurable and very easy to read. It uses different colors for different data points, which makes it faster to see the data you want.
• Cyclemeter has more data points (I think) than my Karoo
• Weight. Yes, it might be heavier than some head units, but unless you are leaving your phone at home, you're carrying 2 devices if you use a separate bike computer.
Cons:
• Battery life -- I got about 4 hours of life or so on a full charge (which means I usually carried an extra external battery - thus negating the weight savings)
• No ANT+ unless you get a separate dongle
• Greater risk of damaging phone in a crash
• Cyclemeter does not integrate my Varia radar, so you have to run the Varia app also and toggle back and forth.
The Karoo is great - but I kind of miss the Cyclemeter display. If it weren't for the Varia integration and battery issues, I'd probably still use the phone.