I don't think either of them are really that difficult to kill, but can be very difficult to find.. at times. More than anything, I would contribute that to shot placement.
I shot two very nice warthogs in Namibia with a bow, only to shoot them later with a rifle... poor shot placement. That's not to say that either one of them isn't tough and US feral pigs/hogs can get much bigger in some place like where I live. A big warthog is in the 200+ lb range. Just about every year, we take 2 or 3 200+ lb sows on our lease. That is done with very little hunting.
Don't get a lot of chances to shoot the big boars here as they are very educated and don't like to show their face. Unfortunately our place is too thick for stalking and we must hunt them at feeders.
A pigs desire to live is only rivaled by the cockroach.
One of the warthogs I had to finish with a rifle was shot 1 week earlier. At the time, I was hunting with a 108 lb compound. After several misses at waterholes, we ended up with an awesome spot n stock situation.
I was able to get to 12 yards and the hog was still face first digging in his den. When he sensed my presence, he did a 180 to see what I was, but unfortunately for him, I was already at full draw... but he was facing straight at me.
I had been in this situation before and new he was about to bolt so I picked a spot on his head and proceeded to shoot about an inch under his brain. He took off with my arrow and luminok glowing. The trackers had a good laugh about the pig with a headlight! We searched and searched... and searched but only found a small amount of blood so we moved on.
As I said, 7 days later while hunting the last evening for my favorite African game species (warthog) we spotted him eating along side the road. Of course he had broke off the arrow so we didn't know that it was the same animal. A quick shot with a .300mag through the shoulders and a short blood trail and we had him.
While cleaning him up for pictures, the tracker noticed that there was old blood and not mud all over his face and a small skin growth covering the broadhead hole.
The skinners pulled my broadhead out of the pigs shoulder. The arrow entered its head just a touch below it's eyes and traveled down it's neck in to it's shoulder. The shaft was broken just under the flesh of the skull and again in it's neck.
A memorable and formidable critter at the least, BigJim