well propietary 5g implementations.. each carrier is doing 5g slightly different than others.. same went with 4g lte and prior..Can you give me a sample of this "carrier customization"? Fundamental to carriers or to us the customers?
also certain features.. sprint for example took forever to just have even a few devices that could use data while on a call..
theres also a difference in overall technology (although this isnt much an issue nowadays) such as cdma and gsm..
you get the idea..