Are you certain that the chimney is completely unused and is not connected to the furnace, water heater, etc. etc.? It would not be good to tap into it and discover carbon monoxide coming back through the other way. Cause, well, the usual way that is discovered is when they find the bodies.

Is it being used to vent plumbing? I've seen people do this in old houses as well - and you don't want methane gas backflowing into the house if something goes wrong someplace, either.

Does the chimney have a functional cap? Can birds or anything else get in through the top? There won't be enough extreme heat to discourage them, but there could be just enough warmth to entice critters into exploring a cozy new home. And exploring all the way down to your vent pipes...

Often old chimneys that are unused get plugged up with mortar or debris, sometimes on purpose and sometimes accidentally. If you haven't already checked for that, you need to do that too.

If all is well and everything is safe and functional, I think in terms of smell, you have to consider one other factor: weather conditions, especially if temperature inversions occur. I am not talking from experience with cannabis, as I have actually NO idea how much the plants could smell since I've never smelled a large number of plants - but I am talking from experience with people who burn cheap coal and other things in their fireplaces or furnaces. Under certain conditions, cold air ends up aloft - in other words above the warm air close to the ground - and "caps" the ceiling, making all the smells remain low. Walking through a neighbourhood where I used to live, you could pretty much tell what every individual person was burning in their fireplace or what fuel they used for heating.

I don't know how much you are planning on growing but I would think a chimney would require the same precautions as you would use for venting anywhere else out the side or roof of the house. Industrial chimneys are a heck of a lot higher than a house chimney and you can still smell what comes out of them sometimes. I'd just apply any of the principles and precautions that someone venting through a roof would apply.