the manifolds were done for a reason... what that reason is, I'll probably never know! LOL But very very few of the people I interact with are burning up-pipes unless it's a truck that pulls heavy. Even see it on stock trucks. Nitrous and really hot tunes like to burn them up. But I just don't consider it an issue for what I put trucks through
the only trucks I see with valve-train issues often have other evidence of infrequent oil changes... they have a lot of blowby, and there'll be the story of "yeah, at 150k, we did the oil cooler... then at 175 we did the turbos... then they replaced a rocker arm and pushrod... And it was good for a long time after that. Then at about 260k it lost a lifter and it completely trashed the engine... that truck was the biggest POS ever"
What they don't tell you or realize is that they were changing the oil every 10k miles by Ford's light duty recommendation (that stretches to 12-15k sometimes after the "newness" of the truck wears off, and you pay less attention to it. Or worse, they're running some wacky extended drain interval.
When things started falling apart, maybe they happened to occur halfway in the middle of those long oil changes, so it actually got some decent service intervals for a while.
You have a stretch of it running well, and you start slacking off on the oil changes again, and a lifter goes.
And they blame it on the engine.
It's not pappy's ol' IDI. You can't change the oil every 15k miles and pour the drain oil in the fuel tank.
The 6.4 was a BIG jump in tech advancement from the 6.0, and the buying public and the OEM and independent service shops are still left chasing their tails and/or just throwing up their hands and saying "it needs a motor"
I digress.
Bottom line? If you treat it right, idle it as little as possible, focus on clean tunes, change the oil every 5-7k, and stay on top of all your other service intervals and only use OEM quality/spec parts/fluids/etc. (the only way I do ELC is if I am building an engine from scratch and start with a new cooling system (I'll leave the heater core) The way I run trucks, I've never had a problem with Ford's Gold coolant
)
Anyhow, you do all that? It's a 400k mile engine. I've seen them with that and more. They're few and far between FOR SURE, but I've seen a good amount of them. Every one of them received impeccable maintenance, PM's, and operated by an informed/educated owner/driver.