What makes it concerning? Tylenol is concerning if you take 2.5x the safe dose. Of course the efficacy is unclear — it’s still in trials!

There was a pretty high severe systemic adverse rate in the high dose group (20%). Severe local response rate was 5% in the middle dose. [1]

I have no idea how this compares to other vaccines, and you can pessimistically extrapolate a bit from this tiny, healthy sample from the high dose (n=15 in middle dose group).

I'd imagine some % of people when this scales to millions will have a bad response, and the high dose group might provide some model of the worst cases.

[1] https://www.nejm.org/doi/full/10.1056/nejmoa2022483

I think the comparison most are making is to Shingrix. Have a look at Table 2[1].

[1] https://www.nejm.org/doi/pdf/10.1056/NEJMoa1603800