I bought an FM bufferless 11.5" kit from Optics Planet (OP) back in December or so. It was shortly before they started offering a .300 blk version. OP was kind enough to accidentally send the same shipment out twice and only charge me once... lucky me -- a spare upper to play with.
I spent the $7.50 for a spare recoil spring assembly (2 springs + guide rod) from OP around the same time and haven't touched them. Each upper came with a pair of extra extractor pins. Haven't needed those yet either.
I converted one to a 9" .300 blk (Faxon), and have had no issues with the upper thus far. I run it suppressed with an standard low-profile gas block. I only have about 200 rounds through it.
I converted the other one to a 12" .350 Legend. Again, a non-adjustable gas block. That one has fewer rounds through it.
I've only had a couple of complaints thus far. I recall their choice of screw head for the handguard crossbolts really pissed me off, and as well as how their handguards ride so close to the uppers that truing the upper or over tightening the barrel nut, even just slightly, alters the vertical alignment of the handguard/barrel. I didn't like their barrel nut, but that's because it is proprietary and requires modification to work with most other barrels.
I never fired them with their original barrels, so my experience is not reflective of an off-the-shelf purchase. I bought the upper knowing I was using it for a project, so, take that into consideration with reactions.