Yeah, I hear you and it is a concern I had myself with some of the changes this offseason but I just keep reminding myself that Berube is going to demand them to be at least defensively responsible. I mean look how far a guy like Vladi has come over the years. He definitely isn't a specialist but I wouldn't say he's a liability either.
I don't think they'll be a big bag of negligent slop regardless of what the roster looks like, but in tight games it only takes one or two mistakes and that can be the game. That can take the form of a critical brain-fart, or trying too hard to make something happen, etc.
The Cup team had a special blend of not making many mistakes, having a defense and goalie that could soak up most of the few they did, a game to game resilience, and the mental ability to trust their game regardless of situation.
That doesn't necessarily need to be the defining characteristics of this team for them to have success, but it sure wouldn't hurt to have them. There's been some key roster changes, and a number of pairings/lines will be having slightly different dynamics, and it doesn't take much to lose that special something that made things really click.
I'm not overly concerned at this stage...more just curious.