dealer technicians see a disproportionate amount of failures... they also see a disproportionate number of trucks owned by non-enthusiasts.
the 6.4 does not have a poor radiator design. Certainly not from a cooling standpoint. So I'm not sure where you got that. the 6.7 radiator is essentially the same design.
techs like the 6.0 because they're easy to work on (compared to a 6.4 or even 6.7), they don't implode as often as a 6.4, and they're far cheaper.
The biggest problem with the 6.4 in stock form is extended oil drain intervals and/or excessive regen cycles causing poor oil quality that eats up rocker arms at best and lifters at worst. Same rockers and lifters as the 6.0's, but regens can murder oil quality and that's when those items can cause problems. (A shelled lifter in a 6.0 or 6.4 = complete engine overhaul!)
in non-stock form, the biggest problem on the 6.4's is cracking a piston. Comes from a lot of timing and heat. it doesn't take much. bone stock trucks can crack pistons (though I wonder how truly "bone stock" they actually are)
one injector nozzle hole aligns with the lip of the piston bowl with advanced timing and will start to crack the piston. once it FINALLY breaks through across the piston, that one nozzle hole acts like a blow-torch on the cylinder wall.
IF the truck is shut down as SOON as the piston cracks, it can be overhauled fairly inexpensively. International has a revised piston bowl design that prevents this issue. It was introduced after Ford quit with the 6.4. It is only available in standard bore.
The aftermarket offers "de-lipped" pistons.
One of the big problems with the 6.4 is almost everything is a cab-off procedure. Yes, there are guys who do turbos cab-on... There are even masochists who have done heads cab-on. But book procedure for turbos, oil cooler, injection pump? all cab-off procedures.
Whereas the 6.7 those are all cab on...
The 6.4 comes out of the front of the truck fine, but because the cab was designed to come off, it's easier to just pull the cab if you have the means.
6.7 cab comes off just as easy.
The 6.7 is a tank. It rarely has catastrophic failures. And if it does, it's all Ford, so you don't pay the double-OEM markup of International made parts. And it's a current engine that's under warranty, so parts are cheap. Wholesale shortblock is $3500 from Ford. I've bought complete good running pull-out engines for $2000-3000. You can expect to pay double that for a 6.4, yet... I wouldn't trust a pull-out 6.4. We only buy Ford remans because they warranty labor as well as parts. But they're getting close to $6k for a reman short block. Reman. not new like the 6.7's
But... with all that said? I know this is an unpopular sentiment, but I think the 6.4 is the better overall package. The trucks are less technologically advanced, and I see fewer little glitchy electronics issues. That's not to say that the 6.7's have major issues. Compared to a Dodge's electronics issues? It's essentially a non-issue.
The 6.4 is a far stronger engine. The rods are way stronger. The block has a bed-plate. The 5R transmission is superior to the 6R in my opinion. It's painfully easy to overhaul, and a fresh factory-spec 5R behind a 6.4 will hold up to 600rwhp for a surprisingly long time unless you're just beating the tar out of it. And I can get a bench job done on a 5R for next to nothing. I can drop the truck off and have a fresh trans for under $2k.
You're going to be doubling that or more for a 6R that will hold the same power, and the 6R's seem to have a mind of their own at times. Not often, but FAR more often than a 5R in a 6.4!
So you have a stronger engine and a stronger/simpler transmission. (people will argue with me on the 5R vs. 6R thing, but there are plenty who agree with me, and they're right, just like me
) AND, you're paying $10-15k less for the truck...
Now... if you crack a piston and torch a cylinder wall, you're going to spend every penny of that $10-15k to bring the truck back on line with good pistons, etc.
But you're still going to have a truck that will hand the average 6.7 its own ass in a drag race or a towing race
With that said? The wise decision is to just bite the bullet and buy the nicest 6.7 you can afford and enjoy the more comfortable interior (the 6.4 interior is NO SLOUCH) and the lack of concern over catastrophic failures
I apologize for the novel, but I'm currently embroiled in the same shopping dilemmas... I've owned four 6.4's. I currently own one (my reg cab) and I'm looking for another crew cab. I've been looking for the right deal on a 6.7 for SIX MONTHS (I want an oxford white long bed w/ black interior... hen's teeth) and I've just recently started shopping for the right deal on another 6.4.
I'll say this. If you aren't prepared to work on a 6.4 AND/OR you don't already have a SOLID mechanic that you know and TRUST to work on a 6.4? Spend all the money and just get a 6.7! LOL