Advantage, whatever the source, isn't a threat to Bounded Accuracy because it doesn't stack and is lower-impact at the extremes of very easy and very hard targets.
It seems like damage, and not Bounded Accuracy, is what's at issue.
I agree. In practice, what Bounded Accuracy means to me is that players will hit most of the time (70%?) against most targets, unless you're fighting a creature known for its exceptional AC. If you add Bless, Advantage, etc. to that, the best you can do is get it up to 95%, which is a meaningful but not huge increase in damage.
What keeps damage bounded is the fact that the maximum potential damage doesn't vary that much between different classes and builds. GWM and SS completely break that, typically more than doubling the potential damage of characters using them.
That's borne out in the table I posted earlier, where adding Bless and Advantage to a baseline fighter results in only a moderate increase in damage, but using them with SS creates a huge multiplier.