The more common valve train failure is the Gen 2 plastic guide, it can fail at the pad where the tensioner pushes on it. Ford updated the tensioner and went to a metal guide on the Gen 3
Gen3 Metal Tensioner Arm - Driver's Side (LH): ML3Z-6K255-A
Gen3 Metal Tensioner Arm - Passenger Side (RH): ML3Z-6K255-B
Gen3 Right Timing Chain Tensioner: ML3Z-6L266-B
View attachment 466460
If the engine spends a lot of time at rev limit or if the engine is seeing high boost levels and a worn tensioner, the plunger on the tensioner can push itself into the pad on the back of the guide (red arrow) and bury itself which causes the chain to get slack. I will try and get a photo of a bad one
The hl3z lifters and followers (rockers) are also not as robust as the bl3z. They do break, seen some posts on here recently even. Probably an issue For higher abuse levels with a side of increased rpm