Ford is the last of the Big Three to have switched to a fully boxed frame. Having driven and worked on pickups with both conventional C frames and fully boxed ones, I prefer the conventional frames, for a couple reasons.
1. C channel frames don't hold nasty crap in them as much as boxed ones do. If moisture, dirt, and salt get inside a boxed frame, sayonara; that frame won't be long for the world and there's no way to know that it's happening nor is there a way to get the crud out. C frames are literally open books when it comes to corrosion: you can see when it's occurring and then can actually take steps to rectify the situation.
2. C frames are generally made with thicker steel, even if it is milder steel than your typical boxed frame. This means that while the boxed frame may actually be (a lot) stronger when it rolls off the assembly line, the thinner material will eventually corrode and/or weaken at a faster rate than the thicker C frame.
What I find interesting (as it pertains to Ford's heavy pickups, anyway), is that F-250s and 350s towed and hauled with the best of them right up until the "Super Duty" launch starting with the '99 MY. My dad and brother had an '85 and '93, respectively. Both those trucks did some pretty serious towing and hauling, and neither frame appeared to balk at the loads. Then, presumably the "Super Duty" launch in '99 started off with an even more stout frame along with improved, larger brakes. My '03 F-350 has also hauled and especially towed some massive loads, and the truck never so much as gave any indication that the frame couldn't handle the job(s).
Then, in '05 the Super Duties got an even stronger frame and bigger brakes, and the same thing happened yet again in '11.
So, before the '17s came out, two generations of frame and brake improvements came and went on Super Duties since I bought my '03. Now Ford has switched to a boxed frame and (I'm assuming) even bigger brakes than the '11s-'16s. We haven't even touched upon horsepower and torque ratings for these trucks, either.
How much more capability will be built into pickups until everybody realizes that "maybe we've gone too far?"