I think the Patriots' defense genuinely started to look better as time went on. Definitely didn't look good on the first drive, at least up until the trick play, but after that they really started to gel. There were a couple of not so great drives, particularly the one that stalled after a penalty from the one yard line, but overall I thought it was a decent start. This tends to get lost in the offense's early struggles, but it took a while for the Patriots defense to come together last year as well.
All that to say I think calling it bad's a bit harsh. Not great, by any means, but not particularly bad either, especially for the unit's first game together.
While I don't doubt both team's defenses will improve (the Steelers in particular by default), I still don't think either defense looked particularly good. When you get burned multiple times by Darius Heyward-Bey - with a long touchdown prevented solely by the fact that it was Darius Heyward-Bey - you know you have problems. I don't think I need to go into any details about the Steelers' ineptness either: the secondary in particular said everything that needed saying.