Connelly just put out his SP+ ratings, rating BYU as the #65 FBS team with an SP-rating of -2.2.

Opponents are as follows

Based on this, we're favored in 4 games (Southern Illinois, @Wyoming, @ASU, Houston), but given the probabilities, the most likley outcome is ... 4.73 wins.  Which explains why FanDuel put the O/U at 4.5.

Arbitrarily bumping the FCS win % up to 90% (could be higher, but I refuse to believe we're less likely to beat Utah than we are to lose to S Illinois and the FCS data is weak so I made that choice) and simulating the season ten million times with these win percentages gives the following results

For more detail, do note the below numbers - the chart shows the possibility of 12-0 as 0%, but that's due to rounding.  There were actually 21 simulations (of the 10 million) that ended up that way... so twice as good as Lloyd Christmas' one in a million chance

But (without inserting any of my biases on how good I think teams are and going simply by SP+), I can see where FanDuel came up with 4.5.  By his SP+ numbers 45.6% of the time BYU wins 4 or fewer. and 54.4% of the time they get 5+ wins.  29.2% of the time, BYU ends up bowl-eligible.

Note that this simulation is not dynamic.  In reality, if we win or lose in a given week, it will influence how good SP+ thinks we are and change our odds for future weeks.  This analysis assumes that the preseason predictions are more or less accurate (otherwise I'd have to sim EVERY team's seasons to see how they all change as well)