Piercing a Na-ion cell is not good, but the effect is pretty much the same like piercing a Li-ion cell.

In both cells the electrode that stores alkaline metal atoms has high reactivity, but in both cases the reactivity is much smaller than for a compact piece of metal, so the reaction with substances like water would proceed much more slowly than in the movies when someone throws an alkaline metal in water.

If you pierce the cell, but the electrode does not come in contact with something like water or like your hand, nothing much happens, the air would oxidize the metal, but that cannot lead to explosions or other violent reactions.

The electrolyte of lithium-ion batteries is an organic solvent that is very easily flammable if you pierce the battery. The electrolyte of sodium-ion batteries is likely to be water-based, which is safer, because such an electrolyte is not flammable. It would be caustic, but the same is true for any alkaline or acid battery, which have already been used for a couple of centuries without problems.

Overall, sodium-ion batteries should be safer than lithium-ion batteries, so safety is certainly something that cannot be hold against them.