Honestly, it's not as noticeable a difference as I would have guessed.

In the last decade BYU has played 15 home games against P5, 24 Away, and 3 Neutral Site

6/15 at home (40%) -> 2/6 (33% against "good" teams), 4/9 (44% against "bad" teams)
10/24 away (41.7%) -> 3/14 (21% against "good" teams), 7/10 (70 against "bad" teams
0/3 Neutral (0%)

The biggest takeaway here is that BYU looks pretty good (in a small sample size) in away games against bad teams. Frankly, I'm not sure what to make of that, but am tempted to chalk it up to small sample size weirdness. Maybe BYU gets pumped on the road though. The things we're definitely not seeing is that BYU has been exceptionally hurt by playing 60% of their P5 games on the road.

Of their 5 "big" wins (beating a P5 team with a winning record against other P5 teams), 2 came at home (2013 Texas, 2019 USC) and 3 on the road (2012 GaTech, 2018 Wisconsin, 2019 Tennessee).

Given that BYU has played 61.5% of their P5 opponents on the road and 38.5% at home (ignoring neutral site games for the moment), it's right in line with expectations to have 40% of the impressive wins come at home and 60% on the road.

Honestly, I would have expected this to make a bigger difference, but it just doesn't. Given this second look, I end up back where I started. BYU, as presently constituted, is similar to a bad (or at least subpar) P5 team. They're not Alabama or Georgia or even USC or even Utah, but they're on the level (or above) of the Georgia Techs, Vanderbilts, Oregon States of the world.

Again, I think this is a compliment. BYU is winning at the level of a low-level P5 without the same resources and would likely improve if they got those. They've won 38% of their P5 games, approximately twice the general G5 rate (from a few years ago - https://www.reddit.com/r/CFB/comments/9buxcv/table_alltime_g5_vs_p5_winloss_records/). But the Home/Away excuse doesn't carry as much water as I would have thought.