It's better if the duct in the unconditioned attic space is insulated, it'll help the evacuated air stay warmer and help prevent held moisture from condensing on the walls of the pipe and running back down into the fan housing during cool seasons. Again, condensation may not be a problem in your locale, with cold attics condensation is more of an issue.

There really aren't any code restrictions on locating a bathroom vent termination hood. Except that they can't vent into an attic or into a crawlspace. Plumbing vents, yes, there are restrictions. Pot/attic vent outlets, there are guidelines for those too.

I'm in New England, with snowfall and roof penetrations in mind, personally my first choice is to vent out an attic gable wall.

If you're going out the soffit, do not let the duct be a blind termination behind perforated soffit material. You can go through the soffit, there are termination caps designed for this. One caveat with soffit venting is that it can sometimes create a mold/mildew build-up on the sidewall of the house and on the underside of the soffit, and the evacuated air can be pulled back in to the attic if you have soffit-to-ridge vents. That's not as big of a deal with bathroom vents as it is with clothes dryer venting, but I thought I'd toss it out based on the stuff I've seen over the years.

If you need to go out the roof, that's a common location, no worries there.

Your bath vent has a certain CFM air movement rating, it's based upon a certain amount of resistance as the air moves through the exhaust duct. If you use ribbed flex duct, if you have too long a length, or too many turns in the dict, the resistance increases and your CFM decreases. So short and straight works best.