Whips are effective (not in terms of robot combat, but generally) because the end flies towards the target, while you already pull the whip back at the same time, causing the tip to accelerate even more (the crack is from getting faster than sound... at least that's what i've heard^^). since energy = m*v², the speed is much more important than the mass hitting. Plus, most whips got a very thin end, and since pressure is N/m² (took these units to simplify) it really hurts when this really fast thing hits you in a really small area.
But that only really works, when you let the whip crack as close to the target as possible. Somehow i doubt you could build a robot doing that^^
Also, the material of the whip would have to be harder than the armor to actually do some damage, and well... building a whip harder than hardox?

Flails and maces were only popular in medieval warfare since they are harder to predict (and control), and also the shock from a hit doesn't get back to you. but they are also hitting way weaker than any solid weapon of same size/weight, because the weight of the hitter isn't behind the blow.