I disagree with the assertion that the Broncos struggle against the top teams. 'More often than not' they actually play well against those sides, even in defeat and it's usually a combination of factors before last tackle kicking becomes a factor.
For instance, in last year's result against the Cowboys, the Broncos were on top with three minutes to go. Milford made an incisive run that busted them open only to lose the ball and give the Cowboys a seven tackle set. Cows make it all the way down deep into Broncos territory, and one of the best defensive players in the game decided to use his legs instead of his shoulders to tackle.
Game, set and match.
People see what they want to see out of those matches. For some it's proof that the Broncos defensive style isn't good enough, the Broncos pack isn't strong enough, the Broncos backs aren't potent enough, this player struggles, that player is selfish. That could all be true, but the asterix that it's apparent against the top teams, or the players struggle against the top teams doesn't hold much weight. It's simply a case of Brisbane losing those games and fans performing a post mortem that hinges on confirmation bias.