Using an old Motorola Defy as Bike Computer
 

So: What makes the waterproof motorola defy + good apps not a much better bike computer then all the garmins? You can take pictures with it, call people, navigate and use it as a regular cycle computer.
It is also (as I said) waterproof, so you can ride with it in the rain no problem. I can't see where this solution isn't just as good as a Garmin (besides the battery, but when do you ride more then 8h+?

GPS accuracy, although that depends on what you end use of the data is to how relevant it is.

It's never as good on phones as a dedicated device, look at Strava, and the users who use phones (esp I-phones) will often have strange high speeds, and glithes/strange paths vs dedicated GPS users.

Cadence sensors, heart rate monitor, accurate speed. I think apps on phones will get there eventually but not yet. You need ant + support on the phone. I doubt you'll get 8 hours with an app running locked to GPS. I run strava on my moto g in my jersey to track rides along with cheap computer to give me speed, cadence and distance in front of my face. With my last android phone I also had some issues with it overheating and spontaneously rebooting on rides with the app and GPS pushing the phones cpu for several hours combined with 90+ temp where I live