Answer:
(These are sample awnsers so I don't recomend copy and paste)
Scientists do not fully understand how stem cells proliferate and differentiate.
There are ethical concerns debated when using embryonic stem cells for research.
Scientists do not know the signals used by stem cells and if they are similar when forming specialized cells; they are unable to grow specialized and unspecialized cells in the lab.
It is a newer technology with many questions left to answer.