I was taught to use groundspeed and it does seem logical and more importantly, user friendly to me.
The purpose of the flare is to reduce the landing speed for touchdown, not to control height, and only an appreciation of the groundspeed can assist you in that decision of 'when to'.
Using groundspeed as your cue, takes away all the guesswork with regard to wind direction and speed, AUW and ROD. Of course, the knack of sensing when the groundspeed is excessive is something that comes with experience thru your initial and subsequent training, but that skill is no different, or more difficult, than many other facets of helo ops.