No ANT+ on a smart phone, and that’s a deal breaker for a lot of my older sensors and HRMs.
Also the bike computer will work under more extreme temperatures and weather, as most smart phones will start to overheat and shutdown at around 95’F, which is easy to do if it is sitting in the sun on your handlebars on a hot day. The Garmin edge has an operating temperature of 140’F, which is way past mine. The same is true at the low temperature end as well where a smart phone ‘gives up’ around 32F while most bike computers operate at far lower temperatures.
Phones are just not made to sit out in difficult environments like on handlebars for a long period.
But to each his own I suppose.