I think it’s false expectations combined with the general problem of being-in-time-for-the-new-strain:
If the people getting the flu shot expect to not feel any reaction, no headache, tiredness, slight fever etc., to the shot, then they will complain of course, but have been misinformed.
The purpose of the shot, as said partly already, is 1) stop spreading the virus - so if you work with elderly, children or the public, it’s a good idea
2) prevent yourself from getting the flu if you belong to an endangered group (elderly, immunocomprised etc.) - that is, if you are able to take the shot.
The concern about catching the real flu is that for healthy people, the flu takes you out one week - not like a normal cold where you might drag yourself in, but one week at least of feeling like full shit, high fever, knocked out.
For at-risk-people, flu can make them die. The Flu epidemic 1918 killed more people than WWI.
In both cases, a slight reaction is therefore obviously better than catching the real thing, but of course worse if you expect no reaction. Which isn’t realistic - there will always be reactions to shots, and mild fever is a very light reaction. So whoever is giving these shots should explain things better - but a doctor or nurse tasked with shooting up 20 people in one afternoon, I can understand them not wanting to take the time to explain in full detail for 20 min. to each single of them. So the paper where people sign up should be more detailed.
The other problem is that the flu virus is mutating so wildly, and the manufacturers of the vaccine are playing catch-up to identify which strain will likely be dominant this year (guessing), then to produce a vaccine to it, then to manufacture and distribute it … all in time before the wild version of the strain can spread naturally. So they might be too late in identifying, or identify the wrong one, or the weather is so cold that people are weakened and catch it 3 weeks earlier than last year …
And if you already have some other strain or normal cold or whatever strains your system, your body will react more to a shot than otherwise.