The audience can lose track of characters and plotlines
Generally speaking, the more perspective characters you have in a story, the more plotlines and side characters you will need to flesh out the narrative. This is a difficulty that can quickly spiral out of control. I recently read a manuscript which dropped fourteen viewpoint characters on me in the space of thirty chapters. That is a lot of viewpoints and characters to keep track of, especially when you only get an occasional chapter here and there with each character. I can tell you the names of about half of the viewpoint characters, the occupations of most of the others, and almost nothing about anyone else.
Now, if I really buckled down and focused, I could probably remember most of the names of the other perspective characters. I could painstakingly map out each plotline in my head to be sure that I understood where things were going. But the trouble is that I just don't care enough to do so. The more work it is to follow a story, the more entertaining that story will have to be to pull readers along; and most of them just can't live up to it.
The audience will almost certainly end up disliking certain plotlines or viewpoint characters
Juxtaposition invites comparison.
|Go ahead. Just try not to notice any differences.|
If you place two portraits side by side, viewers will automatically begin to compare them. If you write a re-telling of Sleeping Beauty, then readers will instinctively compare it to every other version of the story that they've experienced.
The same thing happens with multiple perspectives and plotlines in a story. If you jump from perspective to perspective, readers will begin to compare those perspectives and will almost always end up enjoying or disliking one more than the others. They will begin to resent having to leave their favorite plotline or character to spend time on one that they don't like as much or even actively dislike. In the manuscript I mentioned before, I encountered this problem. Of the fourteen viewpoints I'd been given, I was really engaged by only three or four of them. Perhaps eight or nine of the others felt merely adequate by comparison, and the remaining ones I actively dreaded returning to. If the amount of entertaining story is too severely overwhelmed by boring or unpleasant, many readers will simply set the book aside.
Lack of variety
Many viewpoint characters tend to be something of a blank slate, a "normal" person that the audience can relate to (or project themselves upon) in a sea of distinctive side characters. Harry Potter, for instance, can often come across this way. When compared to the intelligent and bossy Hermione, the sardonic and insecure Ron, the bombastic and friendly Hagrid, and so on, Harry is comparatively low-key. He is interesting mainly as a lens through whom we can view the world and as a vehicle to carry us through exciting adventures, but no so much for his unique or compelling personality. And this isn't a bad thing; a protagonist that your audience can easily relate to or project themselves upon can be a powerful tool.
However, if you have multiple perspective characters in a single story, it can be very easy for them all to fall into this same rut. If you have four viewpoint characters who all have this same, easy to relate to/project upon personality, the story will get boring very quickly. Characters need to interesting and distinct from one another, whether they're side characters or protagonists.
Character knowledge vs. audience knowledge
Unless your perspective characters are all getting together to share their experiences and knowledge every time you switch the viewpoint, you will almost certainly have some characters who know things that the others don't. Your readers and you the author, on the other hand, know everything that any of the characters have learned at any given point. I have read many stories where knowledge began to bleed from one character to another even though they had never had an opportunity to share information. You have to keep track of which character knows what and how they know it; don't just lazily let the audience's knowledge inform the characters.
This also includes keeping track of which characters are acquainted and which aren't. Just because your audience is already familiar with a character doesn't mean the other protagonists are.
Time is one of the most difficult problems of multiple-perspective stories. In the manuscript I mentioned earlier, one of my favorite plotlines was left with two characters in a rather dire situation. The narrative then followed five or so other characters through a variety of situations, including some plotlines that would have taken several days to play out. The narrative then returned to those two characters . . . mere moments after we'd last seen them. I was confused and irritated. If we were just going to come back to these characters in the same scene, the same minute where we'd last seen them, then why did we ever leave them? Days had passed for other characters while seconds had passed for these.
Even more difficult to write and to keep track of as a reader is concurrent action, when two plotlines are occurring at the same time. This can be very difficult to convey, which is why some of the best writers of multiple-perspective stories carefully craft the plot so that no concurrent action takes place. If you leave character A to spend five minutes with character B, then five or more minutes will have passed when you return to character A again.
There are two reasons why this is important. First, if readers get too confused, then they might get irritated with the story or simply set the book aside. Second, there are few things that feel as cheap and deceptive as leaving characters in a tricky situation only to have them get rescued by a character who logically should have been somewhere else. If you make too many mistakes with the time and pacing of your story, you run the risk of seeming dishonest and leaving your readers feeling cheated.
Now, none of this is to say that you shouldn't write multiple-perspective stories. Just keep in mind the difficulties before you start. Don't be afraid to set aside a more complex, multi-viewpointed story in favor of a simpler story with fewer viewpoints that is more within the realm of your skills. Then, once you've had some practice, come back to the more complicated story. Better to wait and do a good story justice than to botch it because you weren't ready.