I think strength of schedule (real, post season SoS based on end of year results vs based on forecasted strength of competition at the time you play them) has to be the prevailing metric in determining who gets in. Otherwise, it will be dominated by the best team in otherwise weak schedules. It is a lot different getting to play Vanderbilt, South Carolina, Missouri, Kentucky (most years), Florida (recent years) and some OOC teams each year....or getting to play Northwestern, Purdue, Illinois, Minnesota, Rutgers, Indiana, Nebraska and Maryland...etc.
I was looking at next year's schedules of teams and there is a fellow SEC team that plays the following schedule next year. They will likely have a losing season, but if they managed to go 9-3 against this schedule, then I would argue they should be in a 8 team playoff. They would not though...because they are not Ohio State, Alabama, Michigan, Notre Dame, Oklahoma, etc.
Arkansas schedule - Top Teams (based on this year's rankings):
Alabama
Cincinnati (playoff team)
Ole Miss
BYU
Texas A&M
Mississippi State
Auburn
Liberty
LSU
Of course they also get to play some weak SEC East teams: South Carolina and Missouri