A monster special ability has a lot of power in its budget; I think 'has no penalty on sweeping attack' for a * is fine.
I think it's probably subtle enough to be decided by the Judge, but I do think it's reasonable to have some sort of requirements to use a 4 stone melee weapon effectively. That's like swinging around a 40-50 lb sack of sand; imagine trying to recover after a miss with something like that, it's not easy. Maybe just a -3 penalty to attacks made with it (-1 penalty per stone of weight above the normal 1 stone encumbrance of a two-handed weapon), or something like that. The automaton system definitely does rely a lot on the Judge's judgement and this is one place where I think the Judge at the table needs to decide what works for them.