There are several that make the widespread adoption now difficult, the first of which is the pitfalls of different phones and the platforms on which they run. Not everyone carries the same type of phone - or even a smartphone. When you have 3,000 people in a company who all need credentials to access a facility, it is rarely feasible to give each person a phone that will run the application needed.
Another consideration is how to handle visitors and contractors that might require short- or long-term access to a facility. Perhaps more obvious is the challenge that emerges when a mobile device runs out of battery, thereby rendering it useless when trying to access a facility.
