I like BCM, just because of their QC. I have one of their BCG's and one complete upper on the way with a free BCG. Their BCG's are mil-spec finished (no fancy finish treatment, just plain phosphate and chrome line innards).
Right now, all 16" uppers in stock ship with a free BCM BCG.
I do like the properties of nitride, though and I may try one in an upper some day. Right now, it's just not on the priority list.
I have a nickel boron BCG that I got from PSA that's been on my suppressed M4gery for the last 500 rounds. It's black instead of silver now (from carbon 'n what not), but it keeps on trucking. I'm thinking about running it until it does jam, just to see how many rounds it takes to cause an issue before cleaning it..
It was $99 on special from PSA.
I have Colt BCGs, DPMS BCGs, and a Young's Manufacturing BCG. I haven't had a problem with any of them.
I think a couple of years ago, James Yeager on his YouTube Channel tested a Fail Zero bolt with no problems. I personally have never used one except for an LWRC upper I used to have and I have no complaints. I think I did a rapid fire test once on it with no problems.
I am hoping sometime this week I will test my new AR pistol build with a Daniel Defense BCG in it to check functionality of the build.
I used a AIM Surplus NiB M-16 BCG in my Blackout build, $129. I can't tell the difference between it and the Spike's Tactical BCG that it matches. It's coated inside and out, as is the bolt and firing pin and gas key. It wipes clean with a damp cloth, and I run it in my rifle with hardly any lube beyond a light oil coat from cleaning it.
I have 4 colt BCG , a PSA, even a RedXArms. All phosphate . Never had a problem with any of them. 1 of the Colts has over 10000 rounds and on 3rd barrel. Still going strong. Everyone seems to have BCG on sale lately. Oh yeah I also have a Windham Weaponry in a 300 BLK build that seems to be just fine.
I have a Failzero as well as a Colt BCG. I am using the Failzero. Why? Easy to clean, needs minimal to no oiling as well as looking cool. But what really sold me is a test report on the exo coating by the feds which sort of validated the coating. I also have read real life tests by experienced users going 4000 to 5000 rounds without cleaning the bolt with no wear seen on the bolt. See link below.