I believe the concern is with the vaccine leaving the injection site in the bloodstream due to improper administration. I don't know about you but every time I got mine the person didn't aspirate first and it scared the crap out of me that they could be hitting vein. I don't think the evidence is yet clear enough on what could happen for every possible variation that could occur. In fact, my personal feeling is that this could be the cause of such inconsistent side effects across the board.
Edit:
@RAHelllord I posted this in the other thread but it explains what I'm trying to say. It's a bit technical but not overly so
Aspirating a vaccine needle is only done in a few parts of the US and some other nations. Not a single vaccine shot is given like that in Germany, for example. It's an archaic practice that simply hasn't been found to have benefits or detriments so it's basically random how any given nation uses it, and it likely has so little impact either way that nobody bothered running studies on whether it's useful or not.
The video is also saying that it's not been confirmed at all and that everything is just a big old "maybe". But even then, the spike protein can only be created by the mRNA that is part of the vaccine, and that mRNA is used up quickly. The spike protein itself is neither contained in the vaccine, nor can it self replicate, and never enters the bloodstream directly, as it's created inside your body cells, which then get told to kill themselves as soon as they start making those spike proteins. If it does affect the nuclei the cells that were affected will be dead after a few days and the damage can't propagate at all. While there are exosomes that feature that spike in the bloodstream, those seemingly only persist for a few months and then stop being a thing, too. But those spike exosomes also can't enter the nuclei directly and the spike protein stays on the outside of the cells, they don't break off and enter further cells.
And of course, no protein spikes enter the bloodstream by themselves:
https://www.reuters.com/article/factcheck-vaccine-safe-idUSL2N2NX1J6
Also, any time a study shows something in-vitro (meaning inside a petri dish only) take it with an extreme amount of salt, not just a grain. Cells behave radically different in a petri dish because there are billions of other compounds and other cells missing to interact with them. Particularly the entire defense system of the human immune system is not present in a petri dish, which would usually find misbehaving cells very, very quickly unless a few other things go wrong perfectly, too.
The video you linked was actually quite interesting, though he did gloss over a few mechanisms like the forced apoptose after the cells have created the spike protein, something that the real virus would try to prevent for as long as possible but the healthy cell can't prevent, which directly affects how much of a threat either of those two scenarios are.
Edit: Apoptose in particular is important because that causes the cell to split itself into fully contained small "trashbags" containing all contents of the cell. This effectively prevents anything to spill out of the cell, these parts get then eaten and digested by white blood cells, turning all complex proteins into harmless base nutrients they then poop out into your body.