Just got the Johnson&Johnson vaccine on Thursday. No side effects, not even a sore arm*. I have severe ME/CFS.
The last time I got a vaccine -- the DTAP-- was in 2011, and it did elicit a marked flu-like response (fever, malaise, etc.) that lasted for a few days. Since then I have declined to be vaccinated since I'm completely homebound and, thus, my exposure to, say, the flu is very low. But if I did get the flu, it's something I'd probably be able to manage at home. While my risk of exposure to COVID is also very low (the few people coming in and out of my apartment are almost all vaccinated--my young nieces would be the only exception), my risk of being hospitalized should I catch it is not insubstantial. Thankfully this vaccine didn't cause any problems!
*Technically there is still the 1 in a million chance I could develop rare clotting issues over the next week or two but, well, that seems...unlikely.