The challenge system works pretty good in hockey. Both for goals and off sides. But you need indisputable video evidence. Often from different angles and high zoom capacity cameras. And even then there are some calls that just can't be confirmed because of obstructions blocking the cameras.
Hockey is a fast action sport, With balls/strikes everything is much more static. There are built in problems though with late moving pitches, hitters stance, the angle the ump stands at. The question is how close to 100% do they want the calls to be? Traditionally it's always been accepted that each ump has his own strike zone. But that just admits that there is no absolute way calls will always be totally accurate.
The technology seems to be there for the robo system, though I haven't looked at the details in depth. Presumably it would come closer to 100% over the long haul. But it would radically change the heart of the game. However, big mistakes really do affect entire games and have repercussions for the standings over a season.
And what about non ball/strike situations? They allow challenges for safe/out calls on the bases and fielding plays. Those seem to be accepted and are non controversial. So it seems balls/strikes are the final frontier.
I would keep the umps, but really judge them more harshly. And add a number of challenges per game.