Well, “forbid manual reactions in online sections” would be a great way to make sure I’d never bother with one.
And personally I feel that making the system automated by default (forcing the player to look for the option and manually activate it) is precisely the thing that Larian got the most wrong in this partial implementation.
There's arguably no worse way to get feedback on a large scale for a feature than burying it and telling your players "Yeah, you'll have to go looking for it".
Doing the opposite would have worked wonders, on the other hand: everyone comes in contact with the new feature and gets a chance to try it and get the pulse of the system, while anyone who doesn't like it can disable it with few clicks.
If not, then personally I'll have to try and convince a player that takes a hour per turn to not use pop ups for his reactions so our group doesn't have to wait another 30 for him to figure that out also.
Sounds one of these fringe cases that never actually happen. A reaction is a "yes/no" binary choice. It would take a special kind of mental impairment to drag it past two or three additional seconds and it's going to be even less typically.
One may as well ignore a turn-based game entirely with this mindset, because "What if the other guy goes to the bathroom or takes phone calls between turns".
Which incidentally applies also to real-time games where you can hit pause (DOTA 2, most RTS, etc).