it's a bad practice to design appliance in such a way to assume that neutral will have low voltage, because in case of neutral failure in three-phase circuit you can get full voltage there, and there can be a couple of volts difference (sometimes more) between neutral and ground even in normal circumstances
it's better to cut off both live and neutral at the same time anyway, especially if there's no standard which is which. also, as device designer you don't know if it'll be used on a circuit that has neutral and phase where you think it'll go or not. (ie british appliance used on unpolarized circuit, like type F. adapters exist)
you don't have to have three phase circuit to be affected by floating neutral in three-phase substation upstream. in some places in us there are 208v interphase three-phase circuits, which give 120v phase to neutral, which is distributed as a pair of wires as single-phase circuit. this is also normal way to deliver single-phase power in europe, as it's most efficient use of conductor. (from 400v three-phase circuits) in case more power is needed than single-phase circuit can deliver, three-phase circuit is installed
if there's switch on device, it's 2p1t meaning both phase and neutral are switched. if it's permanent, non-pluggable circuit, like lightning, it's okay if only phase is switched (neutral is connected permanently)