The SR25 and the M110 have both been used in combat.#MeToo on the M-14. I've no experience with the FAL.
The rifle I used was actually an M-21 with the UNERTL scope, and it never gave me any problems.
But on one of the forums one of the participants was arguing the virtues of the SCAR over other AR-variant .308s, and stated: "...one of its more important attributes is that it has been used in combat".
I had to smile. (And be quiet.) Both the M-14 and the FAL have been used in combat. The SCAR is a more modern design, and it offers advantages such as being able to change barrels/calibers in the field - but how often do you change calibers of the rifle you carry while you're deployed? (And who carries the spare barrels while you're in the field?)
I think the SCAR is probably a great .308, but in terms of performance I suspect it offers only marginal improvement using issue ammo, if any, over the FAL or M14/M21. In terms of long range accuracy there is a reason why Camp Perry shooters widely use M1As in competition.
You won't find too many M1A on the line anymore at Perry. Even on the long line. Liana Bombardier won the 1000 yard match with an M16 several years ago when she was still on the AMU team. 1x6.5 twist barrel shooting Sierra 90s.