Doctoring genes themselves – genetics follow-up topic #4

The 38th of 50 Genetic Ideas You Really Need to Know, the wonderful primer by Mark Henderson, is titled ‘Gene Therapy.’ Henderson relates the 1990 tale of a sufferer of SCID (severe combined immune deficiency), a terrible, rare recessive disorder sometimes called ‘bubble baby syndrome.’ Researchers were able to remove some of her ineffective white blood cells and infect them with a virus fitted with a replacement healthy gene; when infused with those cells, apparently she showed substantive improvement.
Can gene therapy, whether somatic such as this case or germline (also affecting offspring), save people’s lives? Apparently this technique is not seen as holding much promise but why not? What are the pitfalls? Let me see if I can follow this thread.