I've used IA's LoS rules for more than a year.
I've had no problems with balance or quest rules, and its improved the tactical depth of the game in my opinion, since its easier to defend a critical target from attack (for both heroes and Overlord).
Keep in mind that it does weaken any abilities that affect all figures in line of sight, but it does this for both sides.
But have you used it with the app? I fear that the app can't take advantage of the tactical complexity it adds, which becomes one-sided in favor of the players.