I would say the single biggest issue right now is the cost of an LTE modem and service. Cursory research shows a fully integrated modem is on the order of $44 at small quantities. Obviously cell phone manufactures get that number way down but they also manufacture at very high quantities.

It looks like service is on the order of $0.50 per month per device, plus $0.33 per MB. That's not prohibitive, but it would mean the device would likely have to have a subscription of at least $19 a month.