This is very true, a medieval sword is not a katana, it is not as sharp and much of the damage it does to the victim isn't cutting them and crushing them. yes crushing not cutting. An unclean beheading is not an uncommon occurrence in history simply because of how difficult it is to cut clean through a person. Hair is enough to stop a clean beheading, yes hair, long hair over the neck may seem like nothing at all but that is enough to cause an issue that can effect a beheading. If you think that is impossible remember that silk was used in Japan to stop arrows. So why weren't people running around in all silk armour you say? because it has zero effect on concussive damage/blunt force trauma. So the arrow hitting you could break bone but it wont penetrate deep killing you. A fully grown man in clothing is not going to be cut in half but a mundane sword, it just can't and wont happen. No matter the strength of the person or the type of sword.
You have to learn proper sword techniques with a katana to learn not to get the sword stuck in bone. Why? because bone doesn't cut easy at all and flesh offers a lot of resistance so when you reach bone it is very easy to get it caught. That's with a weapon significantly sharper then the swords used by Geralt. It doesn't take a lot of friction to stop a moving object of little weight. There is a reason why most bullet wounds are not through and through even gut shots. Because the human body offers a LOT of friction/resistance and this is from an object at least a hundred times less surface area than a sword travelling at significant speeds faster and they still get stopped by the body.
Now the above said finishing moves are a staple of the series and I play fantasy for the fantastic not the realistic so I give finishing moves a pass. But lets not confuse fantasy with reality.
Most of this is wrong, same as claims of 6kg swords. A longsword is sharp, it carries an edge and is also higher quality steel than a pattern welded katana.
The blade shape of the katana makes cutting marginally easier to pull off, but a drawn cut with a longsword is just as capable of cutting through non metallic armours, and flesh and bone beneath..
The longsword shape is how it is, to permit thrusting, to penetrate weak spots of metallic armour - fighting techniques included half-swording to bring the tip into joints, visor slits etc.
Ceremonial swords and training swords are dulled &/or thicker sections to reduce injury risk and to be cheaper. Quality military weapons are as sharp as any other sword, will hold an edge well and are nothing like the crap "tested" on History Channel and other "which sword is best" TV shows.
(Which also usually have butted maille when looking at maille effectiveness, and so many other inaccuracies and errors it is a shame the money to make them is wasted. They don't add to people's knowledge, but rather actively make people less informed.)