UCF went undefeated in 2017, then beat Auburn in the Peach Bowl. Went undefeated again in 2018 and still didn’t make the playoffs (lost the Fiesta Bowl by 1 score to LSU).
The difference is that UCF didn’t play anyone those seasons. Only played #20 Memphis in 2017 and #24 Cincinnati in 2018.
FSU this season went undefeated including #5 LSU, #16 Duke, and #14 Louisville. The last 8 weeks, they were #3 or #4 in both polls. They then beat a Top 15 team by 10 points in a Power 5 championship game using a 3rd string QB, but somehow got leapfrogged by the #8 team.