I'll just add in that there has been numerous tests recently of the newer SM-3, THAAD and Arrow showing it engaging BM's. Now mind you, nothing is said about type of missile besides Israel that mentioned it was a launch from an aircraft (the time that Russia detected the missile with their radar).
Besides all of this, majority of the talk about ABM systems, majority of the performance characteristics falls under the same as S-300VM and newer. The US and Israel are pushing for KKV's and thus are aiming for high accuracy, which IMO may not be best solution when dealing with a weapon like Iskander who has decoys and maneuvering. Instead, it should be high explosive like S-300V's missiles are like, where even if it does not hit right on, the explosion from the warhead would guarantee it will hit it (well, more or less).
Is Iskander impossible to take down? Probably not. I bet that if they launch more than one missile from air defense systems, that they will more than likely hit Iskander. Is an Iskander a Scud? Yes and No. Yes in the sense it is a IRBM. No in the sense that it flies a different trajectory and it has subsystems making it significantly more accurate and able to survive if engaged.
Most tests for ABM systems, like Pantsir's against Cruise missiles, SM-3 and THAAD against BM targets and Arrow are all pre-planned and pre-determined. In other words, they know where it is flying from, they know where it is going to strike, thus they can concentrate on its tracking. If a missile is launched from an area that is a surprise to you? Then things may be different.
As of recently, systems like Iron Dome have proven itself to be quite accurate and effective of a system. Dumb bombs are not easy to intercept either, especially if they are launched without warning. Russia really needs to work on its sensor development as the west is really improving theirs and eventually, they may very well be able to counter Iskander without much effort.