There is a bias because alcoholism is the core issue that brings many people here but I know there are successful couples who have moved passed some pretty horrible stuff with relation to alcohol.
I think it depends upon if there are other issues in ADDITION to the alcoholism that predict outcome (success or not that is). (ie. Mental health issues).
But it gets back to how you define 'success'. I grew up in a very volatile household where there was no alcoholism, just uncontrolled rage from a father. Hardly a recipe for a successful marriage but my parents have been together for 50 years. Some would see that as a 'success', but there was a path of destruction and tears to get there.
Not a success in my book frankly.
I think what makes a successful relationship is very subjective. I tend towards optimism and giving people a fair chance though and for any relationship, predicting a successful outcome isn't always obvious.