"unpredictable people" is correct and usable in written English.
You can use it to describe people who do things that are difficult to predict. For example, "Healthcare workers often have to deal with unpredictable people every day."
"The reality is you are dealing with unpredictable people.
In Russia, a country where things are random, chaotic, and often unpredictable, people tend to gravitate toward highly ordered social networks, like VKontakte, the Russian analog of Facebook.
We are afraid that they might take one of our children or maybe they might take me now, because they are unpredictable people.
And the connections between pollution and weather are complex and unpredictable: people at the time understood the link between the volcano and the haze, but not the connection with events the other side of the globe.
But all of them can be taken as examples of how unpredictable people can be when they find themselves in the throes of doing what everyone else is doing at the same time.
In addition, each situation may be rife with more danger than others or involve unpredictable people with unpredictable reactions.
What's also surprising is how unpredictable other people's choices are.
But as I said all along, horses are unpredictable and people do fall off.
But while the costs of chronic illnesses are predictable for large populations, they are highly unpredictable for people and their families.
Because the process is so expensive and juries are so unpredictable, blameless people often settle baseless claims to make them go away.
With the economy still weak and the stock market so unpredictable, many people are retiring later because they worry they cannot afford not to.
