Budget
Out of the $12.5 billion allotted, and $20 billion more estimated, I hope we don't end up with a situation where the hardware is built, software is written and tested (both with simulated and real-hardware inputs), but there are no funds left for actual installation and/or training the FAA staff (paying both trainers AND trainees for the time), plus removing the old stuff.
Projects like this make me afraid of cart-before-the-horse situations, where control towers are left with no functional system (for even a single hour) OR a system that functions but they can't use.