OK--the UCC code specifically says:
• Section 1501.1 Outdoor discharge
– Air removed by every mechanical exhaust system shall be discharged to the outdoors
• Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.
– Exception: Whole house ventilation-type attic fans that discharge into the attic space of dwelling units having private attics shall be permitted.
BUT. BUT. BUT. What this specifically means that you cannot run exhaust INTO (inside) the soffit itself. If you do, you're dumping your exhaust inside the soffit space, not outside the building.
You certainly CAN run the vent THROUGH a soffit. Then the vent terminates outside.
So, then the argument is that your humid bathroom exhaust comes out and is sucked back into the attic by the soffit vents built into most roof/attic systems.
BUT, if you have a vent several feet away (10 feet is required for noxious exhaust, but bathroom exhaust is not considered dangerous and not subject to this), you do not have a problem, code-wise or not.
So, the final argument is that your humid exhaust will linger in the eave of your soffit/roof and eventually rot it out. The easy answer to this is getting an end piece or vent that discharges sideways away from the house. Hardware stores have these or can order them--I'm not sure I'm allowed to put a link here.
So, if you consider these and plan for it, you're good.
I personally believe that even a regular round vent in my soffit does not rot my roof during the once a day or so use for a few minutes. I think outside evaporation takes care of that fine. But this last paragraph is just my opinion. I'm much more concerned about making sure that an insect screen or the equivalent is in my vent.