I never went to Cabarete myself, so I don't know how consistent the winds are.
One one side you are right by saying you never have the same conditions. But maybe there they have amost the same conditions.
On the other site you can judge if a kite flies good or bad, has low or high steering forces, turns fast or slow and if the accessories are ok.
Same with a board, you can say if it has a good grip at the edge, goes through the curves like on tracks, pops out good and is easy to play around.
But one thing you never know:
if a testrider personally likes the brand or not...
To avoid this you need as many testers as possible and then the results will be more specific.