Two different points there.You need dedicated radar system to track low flying drones. S-300 and Patriot radars have a tendency to filter them out as background noise.
Anything low flying can be difficult to detect, drone or not. Hence the success of low flying attack helicopters.
As for the drone part, it may be true for very small reconnaissance type drones. TB-2s are big enough to be picked up on radar just as easily as small manned aircraft.
A cruise missile would be a lot quicker and have a bigger payload.Considering loss of two drones per two targets in that particular raid - sounds more like a really expensive cruise missile.
TB2s are cool, but other than moral value, that particular use case isn't exactly inspiring.