Another issue is, other targets may not provide neutralizing immunity and may facilitate ADE. I don't think we've seen ADE in natural infection yet, so it may just be a theoretical issue, but I also don't know if we've really looked for ADE in natural infection either.
We do know that other similar coronavirus vaccine candidates (a candidate for SARS-COV-1) exhibited ADE in primate studies. So since the spike protein appears effective currently, maybe it's best to stick with that.