What I like about the Matthews, Marner duo is they can eat up a lot of time on the clock in the offensive zone before they create a scoring chance, so it has the effect of wearing out the other team, burning up time and putting points on the board.
When it comes to the Tavares Nylander line, what I don't like as much is how other teams can key in on Tavares at the center ice position, which kind of bogs everything down into this scrambled mess. You'll notice a lot of Tavares vs. 2 guys puck battles, and I find the puck isn't being moved around the ice as efficiently. I suspect this style of play is why Tavares had success with more grind style wingers on the Islanders. I think he does end up on the wing at some point and is allowed to ease more into a JVR type net front presence. I don't like him being a primary puck carrier, don't like him in open ice unless he's allowed to build acceleration from way back and allowed to trail on the play.
From a contract POV, I don't love the fact that I'm always wondering how to best utilize him and work around his limitations. While he's doing well statistically, I feel like he isn't the kind of player you can say "we need a boost, let's put Superstar Tavares out to dominate for a double shift." It's more like an arm wrestling competition, if that makes sense.