Yes you can vent into a soffit though the more turns the more backpressure and louder it will be. If the stove is on an exterior wall, I believe the best option would be a 90* turn and vent right out of the wall. The eve should help protect it from water.

Vent out the wall, and have a small angled rain hood on the outlet.

Mine vents directly out because our contractor and framers forgot to bring it out the roof (we lived on site and didn't catch it either). It's pretty nice this way. The unit works better, there won't be any roof leaks ever, and to my relief the hot air blowing down from the hooded exit does not damage our plantings.

BTW, we continued the flue cover to the ceiling anyway because I still wanted it to break up the flat wall between banks of windows.