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The Wizard tracked a little over 2,000 NFL games and found that the home faves were well over bet against the spread
he has the away underdog winning 53.75% of the time ats and generating a 2.57% positive return - see link
I wanted to see more data re that so I found a site with historical data and I found that in the last 50 seasons against the spread away underdogs have won more games than the home favorite in 34 of those seasons and lost more games than the home favorite in only 16 of those seasons - see link
it looks like a player betting only away underdogs will very likely have one of these three things happen - 1. a tiny loss, or more likely______ 2. about breaking even or______ 3. a smallish win
all the while the player can easily be milking free play and/or bonus money while he's doing this
https://wizardofodds.com/games/sports-betting/nfl/
https://www.sportsoddshistory.com/nfl-game-odds/