It's a funny list of reasons, because Real Madrid have a lot more upside in every single one of those. Of the successes of German teams, for some reason I only remember Dortmund's victory (I think in the quarterfinals of the Champions League); in other cases, Real simply destroyed all German clubs, including Bayern.
Overall, it doesn't matter how Stuttgart plays in the Champions League (I don't think they'll make it into the top 8, but they won't be eliminated right away either, so only the playoffs will matter), they need to improve their situation in the Bundesliga - 6 goals conceded in 2 games is a bad sign.

The last reason from that list should be a joke. Calling Stuttgart could trouble Real Madrid is a joke. Their current stats showed how they're being toothless after Guirassy joined Dortmund. They were losing their sharpness. they have been so poor in bundesliga. also, it's an away game for them
It's a fortune for Stuttgart if they manage to enter the R16. They'll have two intensive weeks. First, an away match against Madrid. Then, a home game against Dortmund.
Real is also not in the best shape due to a large number of injured players, but still, these are different leagues to seriously expect a surprise from Stuttgart. I don't know if Stuttgart will be able to get into the 1/8, but it is unlikely that they will be lower than 24th place, which means they will have two games in the "preliminary" playoffs.