Limits include not testing your experiments on human unless you volunteer yourself. Take precaution while prepping for human recombinant protein.
And under the supervision and taking all necessary precaution you can study all that is important to be explored. For example, interesting research includes cloning a lab animal or generating human organ.
Molecular biology as a theory is incomplete: for many fundamental problems in biology, the present molecular canon provides no theoretical predictions and therefore cannot serve as a source of testable hypotheses in these areas. The conceptual boundaries are of two... https://onlinelibrary.wiley.com/doi/pdf/10.1002/bies.950030102
The most serious limitation is that molecular techniques fail to discriminate between viable and inactivated viruses even though inactivated viruses pose no threat to the consumer and may be present at levels substantially higher than the virulent forms. https://pubmed.ncbi.nlm.nih.gov/10382664/
Molecular diagnostic methods offer a faster and more sensitive alternative to traditional immunoassays and culture techniques. Nevertheless, in spite of the unquestionable advantages a quick diagnosis brings to food safety, so far they have replaced traditional methods only in a limited number of analyses. There are still some factors that hinder the adoption of diagnostic assays based on the pathogen’s nucleic acids, rather than on its phenotype. https://link.springer.com/article/10.1007/s12263-008-0106-1
Molecular biology or molecular biology studies biology at the molecular level, so it interferes with both microbiology and chemistry in several branches and intersects with biochemistry and genetics in several areas and disciplines. Molecular biology studies the various interrelationships between all cellular systems, especially Relationships between deoxyribonucleic acid (deoxyribonucleic acid), ribonucleic acid (RNA) and the protein synthesis process, in addition to the mechanisms that regulate this process and all biological processes.