Currently:
Hammerheads hit on 3+
Longstrike also gives them +1 to hit
Longstrike hits on 2+ natively with +1 to wound rolls, for basically 2+/2+ before rerolls.
Some of those rules may have since changed of course.
More worryingly is how many other units have access to railguns that ignore invulns. Can Broadsides also do it with a bit less damage? What about Tidewalls or any
FW units whose weapons get adjusted in line with the codex?
One shot can be quite swingy and unreliable. 10+ shots alongside getting first turn is a whole different situation.