It was described as trying to hit a bullet with another bullet, but in this case both of them are super fast ones. Complications I can think of:

1) ever seen a missile being fired and flowing? It isn't a smooth movement, rather wobbling in +- target direction. I presume this is due to turbulence in the air and resulting corrections by guiding software. Double the issue due to target and pursuer facing random turbulence on their own

2) some SCUDs have some sort of simple 'randomizer' in their flight for exactly these reasons - to confuse simpler defense (ie boat-based gatlink types which fires dumb explosive projectiles) - the target is still in aim, but there are random movements around axis of movement

3) sensors are/were not perfect, non-ideal conditions (clouds, night, cold, humidity, dust), bugs in software etc