For me, it's not even about that specifically. Isner's game is utterly predictable. If a player can successfully manage the trajectory of the ball coming down from his ball-strike (especially on serve), they can manage his game. The variety is almost completely non-existent. It's usually a question of whether his opponent can manage the high bounces coming off his ball-striking. His movement is not as lumbering as it used to be but a crisp shot-maker and a very clean hitter of the ball who can find a target with accuracy and hit the ball right past or behind him pretty consistently. He causes discomfort to some players, mainly because of the trajectory of the ball coming off his serves but a good portion of the Tour is fine with playing him.
To a certain extent, you're right about Anderson. He has shown more variety in his game by adding a bigger arsenal of shots to his game (which is what any player would want to keep opponents off-kilter) but for me, in terms of the sheer entertainment factor, I find Anderson to be lacking.
It still depends on his opponent, whether I stay attentive or completely zone out in his matches. He alone, under his own power, cannot keep me interested in watching his matches.