Which problem can cause kill -9 in production application (in linux to be exact)?

I have application which do some periodical work, stopping these takes long time, and I don't care if some jobs will be aborted - work can be finished by new processes. So can I use kill -9 just to stop it immediately or this can cause serious OS problems?

For example, Unicorn, uses it as normal working procedure:

When your application goes awry, a BOFH can just "kill -9" the runaway worker process without worrying about tearing all clients down, just one.

But this article claims:

The -9 (or KILL) argument to kill(1) should never be used on Unix systems