How might L1 retrotransposons be useful as a potential gene therapy agent? What are the problems ...

Because the retrotransposons can be inserted into the genome, you could theoretically add a missing gene to the retrotransposon sequence and allow the cell to insert the gene into a chromosome, hopefully restoring functional gene expression to the cell. You have little control over where the gene will be inserted, so it may disrupt the function of another gene in the process. Moreover, just because it is inserted into the genome is no guarantee it will be transcribed and translated properly, because essential gene control elements such as promoters and enhancers will not be in the same location relative to the gene.