There are a great many things that I have learned over the years about what is a combat worthy firearm and what is a hobby type of firearm. I have seen such huge variations in AR 15 rifles that I really can’t say that some manufacturers of direct impingement firearms are less reliable that gas piston guns. There are some variations in the older design that have machine their parts so well that they really do run on less lubrication and have run in some tests off thousands and thousands of rounds with out failure. I will say that gas piston guns will run cleaner, but are they more reliable than a high end AR15? I don’t think most of us will really ever know the answer to that because there are always variables in how a firearm is maintained and in what climate and conditions it is subjected to.
If I had to issue handguns and rifles to relatives, I’d probable go with a gun that is easy to clean and easy to shoot, if I want a gun for myself, I’ll take the guns that work the best for me. I know I put more effort into making sure every rifle I own has the right parts staked and loctited down. I have broken a few Tasco and Simmons optics, but I have never had Leupold rifle scopes, Zeiss or Trijicon optic fail on me. There are faulty mounts and scope rings out there, but even the less expensive ones will work if you use Loctite on them and don’t over torque them. Over torque screws on firearms can mean broken screws and I have learned that the hard way with a DSArms SA58. The FAL is not a very good gun for using optics, but it can work if you lock everything into place. Cleaning the guns with this scope mounts is difficult, but the FAL can be made into a good DMR gun.