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.

Oh - I should also mention that the way the immune system responds to the bare antigen is different from the way it reacts when it's included with other potential antigens. This results in a stronger response to that particular antigen. It also means that the lethal over-response that sometimes happens will also be different, and could be more or less severe. It also means that the body's reaction to later exposure to the virus might also be more or less severe.

With a hundred million people vaccinated in the US, we now have results for this "experiment". The vaccine is less harmful than the virus by a couple orders of magnitude - which is piss-poor performance and probably wouldn't have been allowed if it wasn't under a state of emergency.