One of the things that is difficult with respect to strength of schedule is that, when the schedules are made, folks don't really know what the strength will be. Some teams schedule what, prior to the season, seems like would be a tough schedule, but then the teams that were supposed to be highly ranked end up not being highly ranked -- or vice versa.
For example, by mid season, Washington had played Oregon and Stanford. Pre-season, that looked pretty tough. By mid season, that didn't look tough, and the media was complaining that Washington had a weak schedule. Yet by the end of the season, Stanford was back up some, and Washington had also played Utah, USC, Washington State, and Colorado, all of which ended up being pretty good.
Fortunately, it's only the final CFP ranking that matters, and by then, how good or bad various teams are is fully known.
The four teams in the playoff all played some pretty good teams. Judging by final rankings, Washington played 4 ranked teams (Stanford 18, Utah 19, USC 9, Colorado 10); Ohio State played 4 (Oklahoma 7, Wisconsin 8, Penn State 5, Michigan 6); Clemson played 3 (Louisville 13, Florida State 11, Virginia Tech 22); Alabama played 5 (USC 9, Tennessee 21, LSU 20, Auburn 14, Florida 17). Then there were a bunch of teams not ranked but still good teams, or teams that could play very tough for a rivalry game or on a given day.