Earlier quoted context omitted.
Nit: mRNA vaccines were used in human trials for cancer immunotherapy starting in 2001, rabies in 2013, ebola, zika, influenza, cytamegalovirus, etc followed. It's not accurate to say the technology had "never been used in humans before" COVID19; but the US FDA had never approved widespread use of it until then.
This raises the question of why, despite being trialed for twenty years, the technology never was deemed ready for primetime until the emergence of COVID. Part of the reason is safety issues. https://www.statnews.com/2017/01/10/moderna-trouble-mrna/ We assumed that the vaccine doses are low enough to not cause any unacceptable health risks, but after finding these vaccines do cause cardiac complications, particularly…
Personally I would speculate the much larger "part of the issue" was that there was never any real need to seek authorization for mRNA vaccines in the US before COVID19, as alternatives for their targets existed. I would also speculate cardiovascular issues in mRNA COVID19 vaccines (which are real certainly) have something to do with spikey proteins in your bloodstream, and one can imagine what a more prevalent spikey virus might do to the same person. But I am not a doctor or medical researcher, just some guy on the internet who reads things; so I would not really put any stock into my speculations.
edit: my point is: it's good to be skeptical of things. Yes. It's not good to share inaccurate information, as the post above yours did.