What are the major technical obstacles to develop gene and cell therapy?

Some genetics disease can be caused by single genes and other disease like Parkinson’s disease and Alzheimer’s disease may be caused by variations/mutations in several different. Gene replacement therapy for single gene is straightforward but with different mutations it will be difficult as it may trigger various responses. Also gene therapy to a specific tissue needs a calculative time duration and mode of delivery and the duration of gene expression. Also growing the sufficient cell population with self-renewing capacity and manipulating the stem cells to produce enough population for a patient is the next challenge.  Main thing is to provide the correct volume of cells to maintain the normal growth or it may lead to a new tumor development.