I don't think most people realize the difference between overall talent levels vs. matchup talent levels. I believe Dallas has as much overall talent as any team in the NFL, but they're still thin in the most important areas that usually make the difference against the better teams, especially come playoff time. Those 2 areas are, of course, the OL and the DL. When Dallas had the best team in the NFL, back in the 90s they dominated on both the OL and the DL. Jerry is the best owner in professional sports but probably the worst GM in professional sports. He has always preferred flash over substance. If Dallas can somehow "cobble together" enough substance they can beat anybody, but there aren't all that many OL and DL studs just floating around out there waiting to be "discovered".
The other big weakness they have, and have always had since Jimmy left is in coaching. I really believe that McCarthy is the best HC they've had since Jimmy left but still leaves a lot to be desired, IMO. Look at Philly, SF, KC, and any other "elite" teams, their coaches are not only really good at game planning before the game, but at making adjustments during the game. Again, I believe McCarthy and DQ are way better than most of what we've seen since the Jimmy days, but there's still plenty of room for improvement.
Bottom line when you're playing Dallas, get ahead early, cut out the big mistakes and turnovers. If Dallas gets a big lead on you early you're probably screwed, but if you get a big lead on them early, it takes away most of the things they do best, ie rush the passer, making Dak one-dimensional, etc. The problem is, SF, Philly, etc., know that.
EDIT: For anyone who watched the Dallas/Philly game a few weeks ago, at least those who watched it with an open mind, I saw something I DID NOT expect to see. I"m one of those people who believes the champions are the best team period, at least until somebody beats them. So, towards the end of that game I saw something that literally shocked me. I saw literal FEAR in the faces of Philly's entire coaching staff. They were running up and down the sidelines, doing their best Aaron Rodgers impersonations, "hey ref he was out of bounds", "hey ref did you see what THEY did"? "Hey ref his forward progress was stopped when he went out of bounds so the clock should still be running", and so on. I had to see that going on several times before I could actually process it. Like I said, IMO Philly is the best team in the NFC until somebody else proves otherwise. That's what shocked me the most. I couldn't picture them being afraid of anybody in the NFC except maybe SF. It's Philly so maybe their coaching staff was afraid they wouldn't make it out of that stadium alive if Dallas won (which could be a legitimate fear I suppose) but I still have trouble believing what I saw towards the end of that game.
Again, just my 2 cents worth.