Probably one reason is because there are so many ways to play this game. Wether you focus mostly on the fighting, are into min/maxing, or not, skill checks etc. If you just play the game, enjoying most of the aspects of the game, without specializing in any aspect, all the class are fine. And you have 4 in the party anyway, so it's ok if not one character shines at every part of the game.
I don't think for example you'll face a problem in Act 3 because of unstoppable. If you have anyone with
in your party for example, you'll be totally fine, and those fights might even be one of the less challenging fights for you in act 3. Your Ranger doesn't have to be the one. Now if you play on modded extra hard, or want to go through it very fast, or want your PC to be the best damage dealer etc. the Ranger might not be the best character (I don't know personally).
