Sandy and I used Bristol on a boat and set it in the yard for about a year an a half year round no cover. We did the same to an older boat with varnish. Based on what we saw with these two boats the UV from the sun will break down varnish much faster than Bristol. No doubt.
The disadvantage of Bristol is the price and the application. It doesn't like to be fussed with and it is more expensive. The advantage is the long lasting polyurethane protection that is an attractive amber clear.
The disadvantage of any spar varnish is the rapid breakdown in the sun. The advantage is the lower cost and the ease of application and you have a choice of true clear or amber clear.
Varnish has been great for many people for years. Results are fantastic if you garage your boat. Keep it out of the light when not in use, and lightly sand and re-varnish every year or two depending on how much you use the boat.