Being in every character's head during a story will likely result in a lack of surprise for the reader. Readers most often do not want to know all of the action or characters' thought before they occur. Readers need to be given incentive to continue reading, and if the writer gives them all of the information through POV, the reader will not have that incentive. They may put the story down and not have any reason to pick it up again.
Many readers find the idea of "head hopping" irritating. Most fiction teachers instruct their students to avoid this at all costs. Head hopping refers to starting a paragraph or scene in from one character's POV and suddenly switching to another POV within the paragraph or scene. Omniscient POV can be used effectively, provided clear scene or POV breaks are indicated (usually by an extra space between paragraphs), but many writers in the omniscient POV do not do this.
Readers can become easily confused if too many character thoughts and actions are given at one time. It makes it hard to follow and hard for the reader to retain the necessary story information, such as who the character is and what the character's motivation may be. Writers need to be extremely careful to avoid confusing the reader, especially if they are writing more than one story and want future readers.
Once a POV is chosen for a story, the entire piece needs to be in that POV for the rest of the story. For newer writers, this can be difficult to achieve. One or two characters may suddenly start to dominate the story, and the POV may stay with those few characters for the rest of the work. This can leave readers unsatisfied, especially if they identified better with other characters. It also makes it difficult for the writer to tie up loose ends before the story can resolve or end.