This is the biggest weakness of the AR15 design that I know of. Roller cam pin should probably be standard equipment.

Watch this video, it's real interesting:
https://m.youtube.com/watch?v=6JmIQXkoog8

The most common cause of failure in the AR15 is amateur/incompetent assembly.
The next most common cause is amateur/incompetent operation and maintenance.

Absolutely.........you must understand how the weapon system operates at an intricate level, only then can you give it what it needs to work for you....and work when you need it to!

Number one for me is easily ammo.

Didn't read the whole thread but wanted to add these problems I've had or fixed for people:

Under / Over gassed ... fixed by switching buffer weight

Improperly Staked Gas Key ... fixed with my MOACS

Unstaked Castle Nut (receiver extension loosens) ... fixed with a punch

Lose Of RDS Zero ... mounting screws loosened ... fixed with blue loctite

Dead Trigger ... fixed by replacing low quality FCG

Extraction / Ejection Issues ... fixed by adding Crane O and black insert with new spring

I've fixed other less common problems for people ...

Idiot over torqued his barrel and sheared the index pin.

Idiot didn't use rockset on screwed GB.

Idiot bought a 458 SOCOM upper and didn't use the buffer that came with it...sheared the buffer retainer

Idiot installed the hammer spring backwards

Just because you can find a YOUTUBE video showing how to assemble an AR doesn't mean you will end up with a functioning rifle.  I've fixed a lot of idiot builds over the years.