We don't know how much neural capacity is required to feel pain or to have consciousness. It still doesn't know what it is, it can not speak, hear, see or reason. All it does is eat and poop. Now you tell me. This is why the fear is justifiable.
Few people can accept that human embryos are just tissue, like cancer, to be taken out and studied.
It is alive and it is certainly a member of our species. Therefore the question arises of how do we handle the left-over embryos. There are hundreds of thousands of them stored in freezers all over the U.S. and the world, and they keep piling up. Some have suggested throwing them away.