It's hard to go with the flow when you're confident that your team's plan isn't going work. But, because offering your point of view without being asked can make you appear insubordinate, it's important to make it clear that you're dissenting because you care about the team's best interest.
One approach is to ask whether your boss is open to feedback. This way if your boss says okay, you've been given a forum to express your concerns. And if your boss explicitly says no, you can file that away for when your boss asks for an explanation for why things didn't work.
Hopefully, as you gain credibility, your boss will start to ask for your opinion earlier in the process. While this takes some finesse, remember that focusing on your team's long-term performance is more important that any particular project.