On the one hand, you can only do 'special' finishers when it's the last enemy and they don't have much health. They're really nice, even if it's just Geralt simply stabbing them in the chest.
However, if you're significantly stronger than a soft-bodied enemy, you will chop their body into pieces with the killing blow alone.
For example. I encountered some Drowners once I had levelled a bit, and when I strong-attacked some of them, their bodies properly got chopped diagonally in half, or their arms off. The dismemberment was glorious.
It was pretty shocking when I accidentally did it to a guy I fought in an Inn, though. I didn't want to kill him but there he was, his body in separate halves across the room.
You can't do it whenever you want and it doesn't always happen, but it happens enough to be a nice special touch and be believable.