As an owner of an F250 V10 4.56 geared truck and also an Ecoboost F150, I can honestly say that the harder I push the F150 the more it impresses me.
I have no idea if the truck the OP mentioned is within it's rating or not as does anybody else since we don't know the details. What I can say is very recently I ended up hooked up my F150 to a heavy frame trailer with a T770 Bobcat, a Harley rake, and a set of pallet forks sitting atop. Naive to the weight, I drug it around for the weekend noticing the only issue being that the trailer brakes seemed a bit lacking and the trailer wanting to push the truck a bit when stopping. Normally, I would have used the F250 for this as it's my dedicated tow rig however I also was planning on picking up an S220 on a much smaller trailer which I expect to weight south of 10k. At the end of the weekend as I was hauling it back to a buddies place and talking to him about how well the F150 handled it, our curiosity got the best of us and we headed across the scale. If I wouldn't have seen it with my own eyes I wouldn't have believed it. 21,700 lbs! The truck itself weighs 6k on the nose, I've weighed it numerous times. Holy good god, get this thing unhooked from me! Heck, the ol F250 isn't rated for that kind of weight either! We unhitched about 15 minutes later with me looking at him saying "I thought you said this only weighed 10-11k, not 16K!
Long story short, considering the situation and the amount of weight I was dragging around, outside of the 7.8 mpg indicated on the information screen, the truck handled it surprisingly well. It drove down the road nice, didn't jerk and bounce all over the place, pulled it in 5th without so much of a hiccup, even being grossly overloaded! I'm by no means recommending anyone do this nor would I probably do it again knowing what I know now. Considering the fact that I've towed trailers 10s of thousand of miles and have hooked up to everything from small utility trailers, to 40' fifth wheels, to double towing, and so on; and being in some harry situations between bad weather, to blown tires, it honestly handled it unbelievably well, far beyond anything I would have imagined.
With that said, seeing 150s pulling trailers such as the OP has noted and something I always noticed and commented on when going to the lake with my F250, I have a little more respect for those setups and for how well they likely drive! They probably aren't as unsafe as one might think. Again, I know I will be condemned for the above, but I am in no way recommending that anyone hook a 15k lb 40ft fither to their 1/2 ton. I simply want to point out that these new trucks are more capable and safer than any of their predecessors have ever been and condemning someone that is suspected to be within + or - 1k lbs or so of their rating is probably unjust (assuming no overweight of axles or tires!). Had I drove a little ways and felt it handled horribly, was unstable, or otherwise; it would have been hooked to my F250 or my fathers F350 dually the second I got home as both sit sided by side in my shop at my house. This is what blows my mind, it simply handled far better than I would have ever guessed!
I've never towed a high profile trailer with the F150 and I'm sure that would have some additional effect on the comfort level. This is just some food for thought when second guessing others. If you see some unsuspecting person towing a T770 on a large trailer with their F150, feel free to warn them of the weight!
Feel free to condemn away (I was WAY over weight), I just wanted to maybe shed some light for those that have never towed with any of the newer generation 1/2 tons. For me it was eye opening. As they say, ignorance is bliss (I'm talking about myself in this case), ha ha.