Being a person that ones both a 1/2 and a 3/4 at the moment here's my thoughts. Local towing or withing a couple hundred miles, I'll drive my F150 3.5 Ecoboost up to about 8-10k lbs. It pulls, handles and stops just fine. The reality of stopping, is any trailer of significance should have brakes of it's own. If it cann't stop it's self, it probably needs some brake work. The newest crop of pickups is absolutely more capable than there predecessors of even 10 years ago. That goes for 1/2 tons all the way up to 1 tons. Food for thought, but my 2011 F150 Crew Ecoboost weighed the exact same as my dads old 93' F350 dually 460 powered camper hauler. Both weigh right on 6k lbs. My 15' F150 is about 300 lbs lighter with the aluminum body. A new 3/4 ton gasser is going to weigh around 7k lbs while a 3/4 to 1 ton diesel will be around 8k lbs depending how they're configured. Once I get over about 10k lbs, I usually pull out the super duty.
I think the biggest limiting factor with pulling a 5th wheel with a half ton is bed length and tires. Most 1/2 tons come with junk P rated or 6 ply tires which make handling terrible with any significant weight.they are quite, ride better, and might even help fuel economy but they sway and squat terrible when weighted down. I run 10 ply on all of my trucks and it make a significant difference in both handling and tire wear. Heck the last set of BFGs came off at 70k miles and would've made it 80k but it was winter and I didn't want to drive around all winter on marginal tread.
The other piece is bed length. Most half tons come with the short 5.5' bed and a long bed is considered a 6.5'ter any more. You'd have to have a fifth wheel with an extend pin box and still be careful with a 5.5' bed or some combination or a extended pin box and slider hitch to have the bed room to comfortably maneuver a fifth wheel. If you go with a 6.5' bed, it would certainly be better and depending on the fifth wheel you could probably make it work.
This all being said, you will be over the weight rating of the 1/2 ton with most any fifth wheel as they typically have about 20% of the trailer weight on the pin and most 1/2 tons don't have a sticker rating for much more than passengers. Personally, I don't get hung up on the door sticker, make sure you aren't over your rear axle or tire ratings, and if it handles comfortably knock yourself out. Last note, everyone will have a different comfort level towing trailers depending on the drivers experience and abilities, do what works for yourself, not what works for others.