Why do some people consider appendicitis as a serious condition?

It is! Left alone it can be lethal. Delayed treatment can be very dangerous or lethal due to infection, peritonitis, sepsis etc. When identified early, surgery is curative and low risk.
It can be. When caught early (prior to rupture), appendicitis is a very simple disease to treat, with low complication rates. Once rupture occurs, many people can get quite ill due to the bacteria from the intestinal tract directly entering the abdominal cavity thru the hole in the appendix. This is even more serious in those with preexisting medical problems.