I think that the 18 games would mean more injuries, and more fatigue later in the season. But how many people were against the idea of a 16 game schedule?
Surely the first season might be a bit hard on the players, but I am sure coaches and players will quickly acclimatize to the extra games by the 2nd season, with some lagging behind and being used to it by the 3rd season.
Also, all rookies have to get used to the extra games in their first season, and are usually fine with 16 games in their 2nd season, so wouldn't the idea just be like a rookie year again, without getting used to the playing style again, just the length of the season?