The main concern about this vaccine, and COVID-targeting vaccines in general, is that the disease kills because the antigens it expresses tend to make the immune system overreact, so presenting the antigen apart from the disease can also make the immune system overreact. The mRNA version is intended to present the antigen at lower levels over a longer period of time than simply injecting the antigen. But it's still producing the antigen, and if your body overreacts there's no way to undo adding the antigen production to random cells in your body, it'd be many weeks before the mRNA degrades and production stops. There are also questions as to what effects the antigen has inside cells. The mRNA itself, not so much. Simply injecting an antigen doesn't necessarily result in its uptake.