Bulkheads together with archer support turn into the dreaded skirmisher+riot combo, with the difference that archers are also considered to be raiders and skirmishers partially work like artillery. In other words, you cannot stay far away to avoid damage, and you can't get too close without taking heavy damage from archers. Bulkheads also seem to be times as tanky than any normal skirmisher unit. That's my analysis of the game so far.

Oh, and another interesting point. Due to the fact that both Bulkhead and Buoy struggle at hitting moving targets, any fast-moving skirmishers should be effective at them, with proper riot support. Due to the fact that Bulkheads become stationary to fire and archers are relatively slow, artillery could work surprisingly well against them too. However, slower units like riot masses and light close-range units like raiders should only be used for harassment/harassment denials.
I took some time to think about the Bulkhead weaknesses to write this. I hope my analysis helps you in the future, TheWarning.