Anecdotally, dozens of people in my social circle (aged 30-50) reported that the worst vaccine side effects were (1) a sore arm and (2) 24-48 hours of sleepy brain fog (even after the second dose). Among first and second degree connections on Facebook, I know of exactly two people who experienced the full gamut of flu-like symptoms, such as fever, chills, and whole-body aches for a day or two. Take this with a grain of salt, but I'd estimate an upper bound of 10% for "moderately unpleasant" side effects among young-ish, moderately healthy individuals.
Anecdotally, dozens of people in my social circle (aged 25-45 or so reported severe side effects, including at least three or four of sore arm, fever, chills, headaches, exhaustion, body aches, nausea, and brain fog. My girlfriend and I had five of them, for an entire day.
I'm not sure if your group's experience or mine is more typical. That's why comprehensive data is required to sort things out, and limited-number anecdotes are useless.
Basically none of which rises to the point of hospitalization and is gone in a few days.
The risk of hospitalization among even 18-29 year olds is not negligible. Neither is the risk of permanent side effects like diabetes. Those autoimmune conditions can and do strike even perfectly healthy individuals.
It is weird how the vaccine is probably 1000x safer than getting the virus, but young people in particular are happy with the idea that "that won't happen to me, I'm healthy and young" when it comes to the virus, while they're deeply concerned about vaccine side effects that really aren't concerning at all.
(And BTW "robust immune system" doesn't help you if your own immune system turns on you due to the virus).
