shell side arm doesn't target the defender's lower defense using slowbro's special attack. rather, it becomes either a physical attack or a special attack depending on the defender's lower defensive stat, which means you can't exactly control whether the move will be physical or special. for clarity, this is the difference between shell side arm and photon geyser, which works the same way but is decided based on necrozma's own attacking stats.
This is why g-slowbro has equal base atk and spatk. in order for shell side arm to do consistent damage as both a physical and special attack, you need to properly invest in physical attack as well, but since you'll be using only special attacks anyway outside of it you get a lot more value from running sludge wave / bomb as poison stab and psyshock to target special walls. This is even more true for nasty plot sets, as already stated.
That is, unless you are running some mixed offensive assault vest set, in which case shell side arm might be more consistent.