The concept of being forced to live for eternity in a body that has been dead for decades and is literally held together by bandages and spray paint is pretty horrific when you stop to think about it. Especially the state they are in at the end of the film. They're stuck like that forever.
Though they seem more annoyed than horrified.
And how about the good folks that would soon be proceeding out of Ernest's funeral only to find two disfigured corpses on the steps of the church talking to each other?
Also, it's inevitable that each and every one of the people who have taken the potion will eventually get killed somehow and find themselves in Madeleine and Helen's shoes.
Consider how far they can break up. What will be left in a century or so? Sentient dust?
Living forever, even after you've already died, having to maintain that corpse even if it's broken into a zillion pieces. Ernest was right to refuse Lisle's potion.
When Ernest is talking on the phone and we see Madeline's corpse staggering to its feet in the background...complete with neck twisted and broken. The background blurriness makes it incredibly creepy.