On small 1v1 maps they are crazy. The most efficient tool. A safe strategy is build 1 mortar emplacement and one AA emplacement one close to the other and both of them close to base, and guard them almost all game while bleeding your opponent that tries to kill them. Then, in lategame, crush him with your tanks. Repiar them, brace them and they will be immortal even if bombarded by mortars and leighs.
That only really works against players foolish enough to engage them early game with a skeleton crew. A smart player will punish this strat by pocketing you with the aid of a light vehicle and simply rush for lategame armor. As demonstrated here: https://www.coh2.org/replay/53144/how-to-anti-emplacements and here https://www.youtube.com/watch?v=eCYlKxaOtgI