Did the disarm range and disable range get mixed up?
It would be fairer if an EMP grenade had a maximum amount of mines it could affect which changed with veterancy. i.e. Recruit Emp > Max of 2 mines, Veteran > Max of 4 mines, Elite > Max of 6 mines, Heroic > No max [Unlimited] | The disarm range could also be reduced for Recruit, Veteran and Elite.
If the buildings' walls could stop EMP blasts then there wouldn't be much of an issue but as it stands so long as an EMP is near the AirTower or the side of the HoN, a majority of mines (if not all mines) are disarmed with just one EMP grenade.