I was in the market for awhile but ultimately decided against a phone mount. I recently got into a run in with a car. Had my phone been attached it would have made a bad day worse as I'm pretty sure the phone would have been damaged. I also find phone screens hard to read in the sun. And then, overall, I realized it's more important that I keep my eyes on the road than have a computer to obsessively check.

All this to say, the advantage of a dedicated bike computer is that it's durable, easier to read in sunlight and won't leave you SoL if it gets damaged while you're out in the boonies. They can be cheaper than a phone too, though the higher end ones are around the same cost (which is crazy to me). BUT, for me, I'd rather not have anything mounted. Just ride and enjoy the scenery and keep an eye on the road for dangers. I'll run my phone from my jersey and review my stats in the safety of my home.