Another similar idiom that's used is "the tip of the iceberg." Here's an example:
Frank: "We only had two users who complained."
Ernest: "Yes, but that's just the tip of the iceberg."
The idea, of course, is that we only see a little bit of the iceberg above the water, but that's only a small indicator of a much more massive, unseen, dangerous chunk of ice below the surface. In the dialog above, Ernest is telling Frank that, even though only two users have formally complained, there are likely dozens more who are very unhappy.
The idiom could also be used like this:
Seth: "I'm hearing a funny noise in my car. I think it's time for some new tires."
Beth: "That's probably just the tip of the iceberg!"
In this case, Beth is agreeing that Seth may need new tires, but is also guessing that the mechanic will find plenty else that needs fixing, too. She might be especially prone to say this if Seth's car is old and falling apart.
As I said, it's similar. Maybe not exact, but along the same lines.