Already been through this before. The only difference between mRNA spike and wild is that mRNA version does not fuse cells. Other than that, they are identical. Thus, their behavior will be identical as well, minus that missing attribute. You cannot create a spike that's genetically identical and have it operate in an entirely different manner.
The spike is deemed to be safe on the sole basis that it remains localized to the injection site. If it remains localized, it is absolutely safe. If it circulates through the body, it is no different other than not being able to fuse cells. That said, there are already studies showing that the mRNA spike can and has circulated.