[randalluk]

RNA is genetic material, but it encodes instructions to make proteins, which form the physical shell of the virus crucial to its function. As a very rough analogy, the RNA is source code and the proteins are the compiled program.

It's often the protein molecules that the immune system learns to recognise and attack.

RNA vaccines work because your body automatically translates them into some recognisable part of the viral protein, and then develops an immune reaction to that.

If a virus had Ψ instead of U in its RNA, it's still going to be making the same type of proteins. I can't see why it would be more likely to evade an immune response.