It’s possible in football to have several undefeated seasons. Oklahoma had a 47-game winning streak under Bud Wilkinson and undefeated seasons in the modern game are not a surprise.
It’s much harder in basketball. UNLV was close in 1991 and Kentucky was very close in 2015, losing to a very tough Wisconsin team in the Final Four, although people tend to forget that Notre Dame was one play away from beating them in the regional finals. And of course Duke beat UNLV in the 1991 Final Four to deny them an undefeated season.
What’s less understood is that even the most successful coaches have nightmare matchups, guys who for various reasons just give them fits.