As a generalisation, positional players cost more than linemen, so having too many positional players will leave you with not much money to buy important things like rerolls and fan factor.

I personally don't like Big Guys too much, especially for starting teams. They cost a lot, which means that you don't have as much money for the rest of your team (including rerolls and Fan Factor), and they're unreliable. All big guys suffer from some kind of stupidity, which means that they won't always do what you want them to. Plus they can't use team rerolls, so if they fail something, your turn is over.