I guess I have to ask whether you are in England or one of the other English Commonwealth countries where you would lable a ''Boom kicker'' what we in the States would refer to a
boom vang. Otherwise, I am guessing that you are using the term ''Boom kicker'' to refer to the Manufacturer of one kind of semi-rigid vang system. If you are refering to the manufacturer Boomkicker, then the advantages are:
-No moving parts,
-A little less cost,
-Less maintenance,
-Less likely to be bend and therefore knocked out of action at sea.
The downside of a ''boomkicker'', the manufacturer when compared to other semi-rigid vang systems is that it can''t be locked in place so a toping lift is needed once the sail is down. Tension cannot be adjusted. When they break they are shot rather than being rebuildable like many of the more conventional semi-rigid vang systems.
I don not like so called rigid vang systems like the ones made by Spinlock as they are of limited utility and, unlike semi-rigid vang systems cannot be easily used to support the weight of the boom in light but variable conditions.
Jeff