Locking / Winning matches with a simple robot design that anyone can build = Bad. Ex: Cagebots in ITZ.
Locking / Winning matches with a difficult and challenging robot design that very few people will be able to build = Okay. Ex: Cagebots in CU.
Hoarding all the goals into the corner and locking down the game = not challenging or difficult.
Possessing all the goals and maintaining possession for the entire match = challenging and difficult.
At some point, the best robot should win.