Hawk Diesel
Adventurer
I don't think it punishes them. They can still cast spiritual weapon and can upcast it for extra damage as well if they are high enough level.
The cleric can cast cantrips or spells with spiritual weapon but that is a normal spiritual weapon, not a sneak attack spiritual weapon. I don't think it is unbalanced for the rogue-cleric to cast spells or cantrips while using his spiritual weapon, I think it is unbalanced if he does than AND gets to sneak attack as well. The only character that can sneak attack and cast a full on 1-action spell or cantrip the same turn is a fighter-rogue who uses action surge or a sorecer-rogue who uses metamagic and those are both limited per LR/SR.
Different strokes for different folks I guess. I mean, as long as you maintain the once per turn limit for sneak attack, what does it matter if the rogue-cleric or cleric-rogue applies it to spiritual weapon or an actual weapon? I don't see much of a difference personally.