mrna vaccine technology should be used to attack viruses that are more stable than the flu or covid, which will mutate at a high rate on a regular schedule. the problem is that it's too specific, so it loses efficacy faster than the oldest method, which i understand is still the best, which is just injecting dead virus.
if you use the whole virus, the vaccine may still stimulate antibodies based on other parts of the virus, even after it's mutated several times. if you use just the spike protein, it will stop working when the spike protein changes.
mrna vaccines may cause viruses to mutate in the specific way to evade the vaccines. that is less random than not using mrna vaccines. i wouldn't expect this is a big deal, though.