These types of assassinations don't seem to happen very often, so there probably isn't much actual data. But I would expect that a single shot of a rifle is quite difficult to pinpoint, especially in an urban setting. There are audio gunfire locators that some cities have, but even then, the assassin can be long gone by the time anyone could respond. Just drive a car into a nearby parking garage, do the thing, and leave. For a drone operation, the setup and "mission" time is almost certainly much longer. I don't think we're anywhere near fully autonomous missions that could assassinate with any appreciable reliability, so you're going to need to fly line of sight (very impractical because of the range) or FPV (still impractical because of the required skill, equipment and mission time).

Could a professional band of assassins pull off an assassination with a drone? Certainly. Would it be far easier to just use a rifle? I'd say absolutely.