The problem with mers and other candidates for Sars cov 1 was that they are still poorly immunogenic despite being complex viruses. Both cell mediated, meaning t cell, and antibody mediated, meaning b cells immunity seems fleeting. The mrna approach is fairly new mainly because mrna can now be synthesized artificially in large amounts. It does have some benefits. No animals are involved. No complex manufacturing steps such as as inactivation etc are required and copies of each vaccine dose are faithful. Problems are multi fold. Besides the fact that the mrna might be coding for a toxic protein, no one really knows whether the mRNA stays in place in an intramuscular injection. The optimal theory is the muscle cells incorporate the mrna transiently, the s protein is made, the immune system makes t cells and antibodies to the s protein. There is no guarentee that the mrna won't get into other cells, the general vasculature, neural cells or other privileged compartments. Anyways let's see.