The Baltimore Ravens and Minnesota Vikings are both 3-0 to start the year, two of just five undefeated teams remaining in the NFL. But given the way that both teams have played so far, there are a lot of questions about how sustainable their success will prove to be as the season continues.

Let’s start with the Ravens. Although 27 other teams wish they had Baltimore’s record, I’m not sure 27 other teams wish they had Baltimore’s team. Being 3-0 is great, but the Ravens have managed to achieve their perfect record while racking up about as few style points as possible.

All three of Baltimore’s games have been close, with the team winning by a combined total of just 13 points. And its three opponents were Buffalo, Cleveland, and Jacksonville — teams with a combined 1-8 record, all of whom ranked in the bottom six of last week’s ESPN Power Rankings. In fact, we noted back in July that the forgiving schedule should help Baltimore get off to a strong start.

What does history say about 3-0 teams that feasted on bad opponents? From 1990 to 2015, 63 teams started the season 3-0 against opponents that, like Baltimore’s foes, went 1-8 through three weeks. On average, those teams finished with 10.9 wins — only a little worse than teams who had beaten better opponents:

OPPONENTS’ COMBINED RECORD TEAM’S AVERAGE TOTAL WINS 6-3 11.5 5-4 11.5 4-5 11.3 3-6 10.9 2-7 10.8 1-8 10.9 0-9 10.0 Does schedule strength matter for 3-0 teams? Opponents’ combined record is through the first three games of the season Source: Pro-Football-Reference.com

So maybe the Ravens’ easy schedule isn’t a big concern, but it is smart to be worried about their low points differential. The Ravens have the worst points differential of any 3-0 team since the 2004 Jaguars, who eventually finished the season 9-7. From 1990 to 2015, there were 119 teams that went 3-0 through three weeks, and only four of them — the 2004 Jaguars, 1999 Patriots, 1993 Eagles and 1991 Bears — had a lower points differential than Baltimore does this year.

If we use a simple linear regression between a 3-0 team’s points differential through three games and its final win tally, we’d expect Baltimore (at +13) to win 10 games, implying that they’d go only 7-6 the rest of the way. Things were even worse for the 10 lowest-ranked 3-0 teams by points differential: That group averaged just 6.3 wins over the rest of the season. So although Baltimore should be happy with its 3-0 record, a low points differential against a cupcake schedule is enough to fuel skepticism about the team’s chances right now.

The Vikings’ success so far is based on equally unsustainable performances. They’ve compiled their 3-0 mark despite gaining only 796 yards of total offense, becoming only the fifth team since 1990 to start 3-0 with fewer than 800 offensive yards. Only one other team this year has played three games and produced fewer than 800 yards of offense: Los Angeles, with 788 — and it’s never a good thing to be compared with the Rams’ lowly offense.

Minnesota ranks last in rushing yards per game and rushing yards per carry, so the running game hasn’t powered the Vikings’ undefeated start. The offense is also in the hands of quarterback Sam Bradford, who hasn’t managed many long stretches of competent play over his career. So how have the Vikings raced out to 3-0?

Bradford has a passer rating of 107.8 this year, far above his career rating of 81.6, but two games of solid play don’t overshadow a career of underachievement. (His 22nd-place ranking in Total QBR also suggests that his lofty passer rating will come down to earth soon.) The biggest reasons for Minnesota’s success, then, have little to do with the offense and a lot to do with a fluky scoring performance from its defense and special teams, as well as a sky-high turnover margin.

In terms of fluky scoring, so far this season, Minnesota has scored as many return touchdowns (three) as it has offensive touchdowns, and that trend is unlikely to continue. Even good defenses and special-teams units can’t produce return touchdowns every week. Last year, Seattle and Arizona were the only teams with at least three return touchdowns through three games, and they combined for just five more the rest of the season. Digging deeper into history, from 1990 to 2015, there were 22 teams with exactly three return touchdowns after three games, and those teams averaged only 3.3 more return touchdowns the rest of the year.

As for the turnovers, the Vikings have an NFL-best +8 margin in that department through three weeks, and that trend is also unlikely to continue. From 1990 to 2015, 74 teams had a +6 or better turnover margin through three weeks. On average, those teams averaged +7.1 more takeaways than giveaways through three games, but they finished the year with a +9.5 margin. That means that over the remaining 13 games, those teams only had about 2.4 more takeaways than giveaways. Some of the Vikings’ early-season success has been built on performances that are simply not sustainable.

The good news for Minnesota? Last year’s Broncos showed that a defense-driven team can still win a Super Bowl, even when paired with a below-average offense. The Vikings’ defense leads the NFL in sack rate, and the run defense is allowing a mere 3.5 yards per carry. So it would be unfair to describe Minnesota’s 3-0 start as entirely fluky, even if that’s an accurate term for how the team has scored half of its touchdowns. That said, the Vikings have just two runs of 10 or more yards this year, tied with Jacksonville for the fewest in the NFL, and zero runs of 15 or more. And with only six passing plays of at least 20 yards, Minnesota ranks among the bottom three in the NFL in that category, too.

In other words, despite their 3-0 records, Minnesota and Baltimore are each playing with no margin for error right now. Eking out close victories or winning with turnovers and returns can work for a short while, but both methods are cause for plenty of doubts about the teams’ ability to be serious title contenders.

CORRECTION (Sept. 26, 11:15 p.m.): An earlier version of this article misstated the number of games that Sam Bradford has played through the first three weeks of the NFL season. It was two games, not three.