I believe it is mostly from this stuff here
https://phmpt.org/pfizers-documents/
From what I can see (I've mostly gone on the bits people have posted on Tweets, etc. rather than try and take it all in) a lot of the stuff blowing up on Twitter largely stems from:
People not understanding the data or testing methodology especially where specific studies/tests/data link into a larger regime and
can't be interpreted in isolation, such as where adverse reactions or deaths are only noted, not attributed to a cause but people are interpreting them as all caused by the vaccine when it is standard A and B groups with a placebo testing. Likewise for instance using data which only looks at the first 30 days to claim the vaccine isn't effective - it takes longer than that to reach peak effectiveness.
And so on and so forth - quite a lot is either ignorantly or willingly ignoring what is actually being said to contort it into something which supports the anti-vaxxers position i.e. using data from specific age groups as if it applies to all ages.
That isn't to say there aren't concerning bits because there definitely are - but if anyone bothered to read the UK government's emergency authorisation documents for the vaccines it isn't particularly surprising stuff (the official message played somewhat fast and loose with the truth in some areas - even if in some cases there is possibly justification for it).
Complicated and the vaccines somewhat skew the picture but with Omicron it is much more common for the impact of the disease to be shifted from lower to upper respiratory tract so sore throat is more common than lung inflammation which is one of, though not the only aspect, which makes COVID dangerous. The net effect seems to be the disease is generally less likely to cause severe outcomes in the fit and healthy population who have no undetected serious medical issues, etc. but it still seems to be almost as bad as Delta in the vulnerable and elderly population and probably partly due to the increased prevalence enabling it to reach places it couldn't before it seems to be taking quite a toll in certain older age groups compared to Delta.
However that isn't the whole story as there are still a lot of unknowns as to the longer term picture especially if people get COVID more than once which Omicron makes more possible with its immune evasion. For instance there seem to be a not insignificant number of people who never seem to quite get back to their old selves after a bout or two of COVID - I know people who used to run marathons who are still struggling to make it through a day at work which is probably the impact of COVID.