Roush had a some valve train failures this year. Carl Edwards was on the scanner at Bristol when one happened. He probably would have won the race.
Based on your comments, I’m sure you realize they are always making the cam profiles more radical - up to the point of failure. And of course the valve spring steel is magical stuff but every micrometer of length can’t be perfect.
Beware of what you hear on TV and remember, the driver can only give an idea of what happened and maybe even more so, know they are live on the air and no driver for any team will give a true indication of what he thinks on the air, unless he wants a trip to the Oval Office for one and also a trip to the team owners office.
I cannot impress more upon you just how “managed” many of today’s racing is and it is all about money, not competition.