Ethical concerns with human ESCs are:

A. Since harvesting ESCs necessitates destroying the embryo from which those cells are obtained, the moral status of the embryo comes into question.
B. Scientists argue that the 5-day old mass of cells is too young to achieve personhood or that the embryo, if donated from an IVF clinic (which is where labs typically acquire embryos from), would otherwise go to medical waste anyway. (The 14-day rule is widely adopted).
C. Opponents of ESC research counter that any embryo has the potential to become a human, therefore destroying it is murder & the embryo must be protected under the same ethical view as a developed human being.
D. Embryonic gene editing causes big debates.