Yeah, all of the above would apply IF they do hit. However, there have been some matches where I've managed to SDI out of Marth's >B and have punished him for it, even when they stop it after they realized that I got out of the attack. At worst, I can get a jab in. At other times depending on the opponent's reaction time, an F-tilt.
Well, this is true to some extent however, good players can bait you into doing your ^B attack which they will punish. Steel's explained this already I'm sure.
For the >B, remember it's only for the Down B variation. Also, the distance isn't THAT great. Also, Steel already mentioned that Marth suffers from lag when you stop at ANY variation, including the first. You're probably better off shield grabbing if you really wanted to get a grab in.