[Edi]

Has anyone tried using a bane spider with a bunch of spider warriors set to guard commander? It'd be interesting to see if they join in on the assassination attempt.

The reason the bane spider also has less stealth than other assassins is that he has leadership. Commanders that have stealth confer the same stealth bonus to all stealth units under their command (so for example a Vanherse with stealth(25) gives all those stealth(0) bakemono archers +25 stealth.