Change makes things tough to predict. A new coach, a new scheme, a rookie quarterback — all of those changes can have a major impact on how an NFL team performs, and all of them are a challenge for my NFL Projection Model, or any model, to account for.My model takes into account hundreds of variables, and while taking a data-driven approach is a sound way to make predictions, football is often chaotic and always unpredictable.Who could have known what Mr. Irrelevant Brock Purdy would become for the San Francisco 49ers? How could one have predicted that then-relatively unknown Sean McVay would turn Jared Goff into an above-average quarterback after a disastrous rookie season?These are just a couple of examples of how roster decisions and coaching hires can make forecasting the future a challenge.So, with that background, let’s look at five of the teams that, in my opinion, the model had the hardest time projecting. We’ll start with a couple of teams in the AFC before moving on to the NFC.AFCBuffalo Bills: 10.7 winsJosh Allen is a known commodity, which is a quick way of saying no one and nothing, including my model, doubts the QB will be great in 2026. So why are the Bills on this list? Well, it's the rest of the roster and coaching staff that makes the Bills hard to project.As it stands right now, the Bills are the model's favorite to win the AFC East (51 percent) and rank atop the AFC when it comes to making the playoffs (74 percent). But with a lackluster receiving group and without many difference-makers on defense, I'm concerned about what happens to this team if Allen sees a dip in production. I love the offensive tackles, and Ed Oliver is great on the defensive front, but outside of that, do the Bills resemble a Super Bowl team? Because that's where they're priced right now, and a scary amount of that optimism rides on the right shoulder and legs of Allen.They've also handed over the big job to former offensive coordinator Joe Brady. Former coach Sean McDermott may have had his flaws, but Brady is a total unknown as a head coach. I am optimistic about him bringing in Jim Leonhard as defensive coordinator, but he hasn't called plays in the NFL and hasn't called plays at any level since doing so at Wisconsin in 2022.Pittsburgh Steelers: 8.4 winsThe Steelers are old, especially on defense and at quarterback, with the soon-to-be 43-year-old Aaron Rodgers under center. As you might imagine, aging curves are also a challenge to predict. We expect players to fall off as they get older, but by how much, and when that starts is tough to forecast.Will defensive stars like T.J. Watt, Cameron Heyward and Jalen Ramsey perform at a high level for another season? How far do they drop off? Can they stay healthy?Beyond age, the most difficult variable to account for is how new coach Mike McCarthy will impact the offense and this roster as a whole. McCarthy probably doesn't belong in the elite tier of coaches, but he is a good one. If he and Rodgers can rekindle some magic from their Green Bay Packers glory days, there is enough talent around them to believe this offense can be above average. Of course, that assumes Rodgers can still handle pressure and hold off Father Time a little longer.If the older players stay productive and healthy, the Steelers will be in the hunt to defend their AFC North crown. If not, well, there's no Mike Tomlin magic to save the Steelers this season.NFCPhiladelphia Eagles: 10.4 winsMaybe I'm a sucker for believing quarterback Jalen Hurts can return to the level of play he showed during runs to two Super Bowls. If he does, it will likely be because new offensive coordinator Sean Mannion proves to be a big upgrade over his predecessor, Kevin Patullo. But Mannion is an unknown commodity, and he's far from the only question mark on this team.The Eagles' offensive line isn't the dominant unit it once was, and it's getting up there in the age department. Injuries have become a more persistent problem, and we all saw what happened when star right tackle Lane Johnson, who is 36, went down with an injury last year. There's some noise in these numbers, but with Johnson on the field in 2025, the Eagles offense ranked ninth in adjusted EPA per play; they fell to 26th without him. Is this unit just one injury away from falling off a cliff? We also must acknowledge that longtime offensive line coach Jeff Stoutland is no longer coaching this unit. That kind of change is something a model has a very difficult time quantifying.The pass rush is also tough to project. Before trading for Jaelan Phillips last year, the Eagles ranked 8th in EPA per play, 21st in success rate and 14th in pressure rate. After acquiring Phillips, those numbers jumped to third, second and third. Phillips is now with the Carolina Panthers, and while the Eagles traded for Jonathan Greenard, his status for Week 1 is up in the air.Given all of this, are we sure the Eagles are going to win 10-plus games?Green Bay Packers: 9.7 winsMy brain is like a ping-pong ball when thinking about the Packers. Sometimes I think they could contend for the Super Bowl; other times I think they'll miss the playoffs. Neither outcome would surprise me.On one hand, QB Jordan Love and coach Matt LaFleur have produced top-five offenses in two of the past three years, per my model. However, the offensive line is in the worst shape of LaFleur's tenure, and while the pass catchers have tons of potential, wide receiver Christian Watson and Tucker Kraft have concerning injury track records. More injuries could be enough to sink Green Bay's offensive production.Is Green Bay Packers offensive line 'good enough' going into 2026 NFL season?Robert Mays and Derrik KlassenThen there's the defense, where star pass rusher Micah Parsons is out for the first month-plus of the season. Last year, his first in Green Bay, Parsons was a difference-maker for the Packers defense. When Parsons was on the field, the Packers ranked 15th in EPA per play but fell to 28th when he was off the field. That's a stark difference, and it's concerning that even when he was on the field, they were just an average unit.The last part of the uncertainty for the Packers is their change at defensive coordinator. Jeff Hafley has moved on to coach the Miami Dolphins, and the Packers brought in Jonathan Gannon to replace him. Will Gannon improve the unit? What he gets out of the defense without Parsons on the field could determine the Packers' playoff fate before we reach October.New York Giants: 7.1 winsWhat are the Giants under new coach John Harbaugh? Are they playoff contenders, or a team that will pick near the top of the 2027 NFL Draft? The play of second-year QB Jaxson Dart will go a long way in determining the answer. He remains a bit of an enigma to project, however, following a promising rookie year filled with injuries.Combine a perplexing young quarterback with a new scheme and a brand new coaching staff, and you start to see some wide distribution of outcomes.And then we look at the defense, which has tons of pass rushers. Brian Burns is coming off an awesome season, and the Giants drafted linebacker/pass rusher Arvell Reese at No. 5. But maybe the piece I'm most excited about is pass rusher Abdul Carter. He didn't register the sack numbers last year, but he was incredible at generating pressure. But will this front seven be able to stop the run? After trading defensive tackle Dexter Lawrence to the Cincinnati Bengals, the middle of the defense is thin, and that can lead to these pass rushers being nullified, with the Giants' defense failing to force the offense into true passing situations.