Avoid stouts? I slow cook so many other things with it I woudn't imagine that's an issue.
In my opinion (worth almost, but not quite, what was paid for it)-yes. For BBQ, the bite of an imperial stout, a dry stout or even an Irish stout would, IMO, be out of place. Basically, anything where that stout bite is really pronounced. In other words-all of my favorites.
A milk stout, oatmeal stout or a sweet stout would, IMO, work okay, though I think (given that you are replacing a beer with a light, clean, not even remotely bitter profile) staying away from that end of the spectrum altogether would be best. Some of the lighter stouts being made by some US breweries would probably be okay too-but, again, given that you are replacing something with very little malt or hop character, subbing it out with something with HUGE character is not going to give you a reasonable substitution.
I love stews and chilis made with stout, but BBQ is a different animal and the flavor profile you are looking for is different. Even the more vinegary end of the BBQ spectrum isn't going to benefit from that stout bite. Though I suppose it depends on how much of it you drink while cooking and how much makes it into the dish too. :>