It is true that both the influenza virus and the coronavirus are composed of RNA. What follows refers to influenza A. However, the influenza viruses are segmented and are negative sense/strand RNA. Messenger RNA (positive sense) is made from these segmented viral strands which code for proteins including the outer coat, inner coat and enzymes etc. Segmented means that each strand approximately makes one protein each. Now why are the influenza vaccines not so effective? Every year "they" try to guess which will be the predominant strain(s). Pigs, birds, humans etc are hosts for these influenza viruses and they can be doubly and triply infected so that possible new combinations occur. The main outer proteins of influenza virus are the hemagglutinin (H) which agglutinates red blood cells and neuraminidase (N), an enzyme, which cleaves off neuraminc/sialic acid. There are 18 known subtypes of hemagglutinin and 11 of the neuraminidase. So when animals including people are multiply infected with different influenza virus strains they can readily mix, since influenza viruses are segmented. It is the outer coat proteins which we need to have antibodies against to become immune. In conclusion, there is no single strain of influenza. In addition, there is antigenic drift among the hemagglutinin and neuraminidase subtypes. Notice that the influenza viruses are listed as H1N1, H2N1 etc. in all the possible combinations. Let me further add that killed or component vaccines are not as long lasting as attenuated vaccines such as measles.
There are good, effective vaccines for many viruses including measles, mumps, the polio viruses etc., so we can see how the influenza viruses differ from these since they are not segmented. By the way smallpox which is a DNA virus was eliminated from the face of the Earth when it was declared eradicated in 1980. The last outbreak in the US was 1949. Most of the people on this forum have not been vaccinated against smallpox, but older people like myself still have a scar on there upper arm from the vaccination. Smallpox vaccinated was stopped in the US in 1972. My children three of who were born in the 1970s were not vaccinated against that virus.
So is coronavirus like influenza A or more like measles, mumps, polio etc.? Coronaviruses including CORVID-19 are composed of single strand of positive strand/sense RNA. Coronaviruses have been known since the 1960s to produce respiratory infections in people. A good thing is that it is not segmented, although there are a couple of strains of CORVID-19 mentioned. There are many different approaches to making the CORVID-19 vaccine, and it is not clear which one will win out eventually. A component vaccine would probably need boosts. I think/hope that a vaccine will be quickly produced by at least mid-2021, but CORVID-19 will not be like influenza virus.