Heritability of modifications.

Poor specificity and understanding of the consequences. Cure HIV and get some other genetic disorder (usually cancers) down the line because the sequence they used messed up other areas of your DNA.

Speaking more broadly, a genetic arms race to invent super-people. We already toyed with that once in the 40s, and it didn’t go particularly well. There’s a small technological gap between removing viral DNA for HIV and trying to insert or change DNA for unknown reasons. Cure illness? Great. Eugenics when we understand very little about the interconnectedness of our genomes? Playing with fire.