Germany have an easy group compared to others. Costa Rica have only about a 15% chance of qualifying for the next round, they are only in the WC because of their easy qualifying group. Ecuador i also think should be dealt with, i don't know whether they can still play well when they aren't in altitude. Germany should watch out for them, but they should be able to beat them. Poland are the only risk imo, they are an underrated side with many top players(not including Wichniarek ) and i think they could pose a threat to Germany. But the Germans should qualify.
But I don't think that after this qualification Poland are underrated anymore, everyone seems to take them pretty seriously.
No team should be underestimated. Last WC Costa Rica played draw with Turkey and struggled only by goals against them. Ecuador beat Croatia at the last WC. The teams have no big names, but this doesn´t mean, that they are easy to play.
Poland will be the main threat in the group phase, no doubt. I also expect a lot of polish fans coming to Germany.