Brady, peyton and Roethlisberger are a combined 47-27 since rivers has been in the NFL. There have been 74 playoff games between AFC opponents from the 2004 to 2018 seasons, and those 3 guys have won over half (38).
Flacco has 9 wins against afc opponents in that time, but hasn't put up elite passing stats.
Of the remaining qbs, rvers has won 5, which is more than any other afc qb in that time.
Brady (23), Roethlisberger (12), manning (11) flacco (9) and rivers (5) are the top 5 quarterbacks in wins against afc opponents. All other quarterbacks have 14 wins combined. 5 of his 6 losses came against them, and two of his wins (both against peyton).
I don't know if that's an argument that helps or hurts him, but he played against some very tough contemporaries. Maybe he doesn't belong in the hall because he wasn't top 3 in his own conference. Maybe he does because he played really well but happened to be in a dominant era for afc qbs.
But before you rush to judgment on rivers - the most winning nfc qbs in that same timeframe are Rodgers (9 nfc wins), brees (8) and eli (6). Maybe rivers was just in the same conference as some dominant teams.