It totally depends on how big the knot and/or penis is, and how rough or excited its owner is. I had a medium-large boy (about 55 lbs. or 25 Kg.) who had been neutered quite young, but he was still surprisingly horny and willing. He had kind of a pencil dick which would get slightly longer but much thicker when he was fully erect.
He LOVED to face-fuck me and we did it quite often! His knot was somewhat larger than a golf ball and I would deep throat him once he was finished humping. He would usually tie with my mouth for 5-10 minutes. When we were done with that, I would stroke myself while he licked me and he would get a nice treat, often as not all over his muzzle and paws.
I never scratched him; I had quite a bit of experience by then doing oral on male doggos much better endowed than he was. He tasted quite nice, sort of clean-creamy.
That's not to say that I haven't taken some damage from a face-fuck, though. Some years ago, I met a neighbor's GSD x Husky while walking in the woods. We found a private spot and I got down on all fours. He seemed to know what was about to happen, even though I hadn't done anything with him previously. He mounted me over my head and started thrusting. I took him in my mouth and he basically went wild, pushing in everything he had and humping madly. The tip of his os (penis bone) beat up the back of my throat badly enough that I was sore for days. He was just fine, he wanted another go afterwards. He had to settle for a hand job, lol.