They are unreliable. This is a great way to go from parkinsons to a brain tumor.
What I've read/heard is that the foetus cells multiplied too rapidly and were unpredictable (i.e., tumors) and adult cells which had already gone through the multiplication phase rendered them as more stable... This would explain why children (whose growing process is ongoing) do better with the foetus cells...BUT my opinion (which doesn't count for much) is it seems if you inject cells from a donor who isn't the approximate age (or growth stage) of the recipient...you're going to have problems.
In the article it states they can attempt to take out the defective cells or control side effects with drugs. How is this any different than going back to square one? They're trying to justify a failed procedure...and abortion harvesting....