It shouldn't be difficult to program at all.
What it would be is potentially incredibly annoying if the player is being constantly prompted to take reactions or not for every character all throughout a fight. That's the annoyance they're trying to avoid. They want combat to be smooth.
That said, I agree completely with you. The way they have it implemented now is crap. Maybe let us "opt in" to the specific reactions we want to be able to decide on and ignore the rest? I get where they're coming from with wanting to avoid spamming the player, but this "solution" doesn't solve anything.
I believe the way to fix just constantly getting pop ups about whether or not you want to use your reaction or not is to make it both a precast and a popup question, so say I want to use Hellish Rebuke, I first have to cast it and then whenever an enemy hits my Warlock, it asks if I want to use Hellish Rebuke against that enemy or not. That way it's not popping up every single time an enemy hits you, only after you've precast it. Might still lead to a lot of popups, but it would certainly be less than just having it happen every single time. There's another game coming into EA soon that handles reactions using the popup message system, so we can always take a look at the way that handles as a test to see if it's really as much of a bother as it sounds or not.
