C frame is more likely to break in extreme flexing, i.e. off road "challenges"
Boxed frame weighs more, but has less flex so less likely to break in extreme "challenges"
In a Ranger your more likely to break axles and suspension parts before C frame would break.
But if you are upgrading suspension and axles then boxing the frame would be a good idea.
Attaching parts to a boxed frame brings a new set of issues
With C frame you are just bolting to a "plate", horizontal or vertical, its still just a steel plate.
With boxed you can go thru the rail using longer bolts but there is limited torquing because you will start to deflect the rail.
You can add holes to get nuts and bolts thru to the opposite side "plate", hole needs to be large enough for socket wrench.
Or leave some areas open, as C rail, where attachment points are needed, if its not too big of an area you will still get the strength of the boxed frame but have access to the attachment points.
Or you can weld on attachment points
Limited flex can make for a better ride and more predictable ride over bumpy terrain