Yeah I really hate how one cinematic trailer was the extent of lore this character has ever had. We need more lore to draw back on for answers about the full extent of his power. This also has to have been something the writers have discussed in the past and hopefully have an answer for. Otherwise anytime something awful happens in this Universe, people are going to ask "why didnt Bard prevent that?" But apparently the cinematic was enough.
Having said that, I dont think Bard actually prevents things from happening per se. I feel like he guides things into the right direction but Bard isnt predestination by any means and people make their own problems, like Icathia's Doom.