Delivery is a major obstacle. DNA, RNA and proteins don't typically get into cells on their own. Typically you use vectors, often viruses or lipid nanoparticles, to get DNA into cells, but these vectors have their own challenges. One of which is that they aren't typically very specific to particular cell types. It can also be tough to get vectors to the right tissue. So most early gene therapy efforts focus on blood disease, liver disease and eye disease because it is easier to deliver to those areas. CNS has also become a popular target because a particular viral vector, AAV9, tends to get into neurons pretty well

But viral vectors are commonly "immunogenic", ie the immune system learns to reject them as foreign after one dose. These are viruses after all, and the body is designed to reject them. Some people have pre-existing antibodies to many popular vectors. And often you can only dose a viral vector once, so if you don't dose it right, or if the effect isn't permanent, you don't get another shot

I can't speak to some of the other more technical problems, but one limitation is that you are limited as to how large your DNA payload can be. But it seems this paper is a big step towards removing that barrier. Which I believe is why this paper is such a big deal (others more knowledgable should correct me)

Also CRISPR works best to cut DNA as of now (which renders a gene non-functional), but it is a bit harder to insert DNA reliably from what I understand. Again, a scientist in this field would much better understand the state of the art than I do, but this is my understanding

Another major unknown is the degree to which gene editing tech causes off-target edits. If your DNA editing tool accidentally snips a gene that protects you from getting cancer, it can lead to cancer. I'm not close enough to the science to know what current thinking is on this risk or how to best mitigate it, but it is very real. More primitive gene therapies did in fact cause cancer in patients (often children)

In many ways sci-fi level genetic engineering is possible. We just don't do it because we don't know the risks, and the risks are huge. We can already genetically modify human embryos, and our toolkit for doing so grows every day. Editing human embryos is a very scary proposition.

EDIT: I will also add that if you are interested in learning about this field, try reading / struggling through a couple scientific papers, really trying to understand every detail. Ideally with the help of a scientist friend. It is time consuming and daunting to get through all of the jargon, but the papers lay out the design, engineering and testing process in some detail. Often they provide the derivation of the mathematical models used and the specific DNA sequences used. You can begin to appreciate how amazing this work is when you get into these details