First of all, “be careful!” isn’t very specific. If there’s a real danger, it makes sense to spell it out. “Be careful with that knife because it’s very sharp” not only gives a warning but tells what action the warning is about and why. But we often don’t say what a child should be careful about. We aren’t very clear.
Second, “be careful!” isn’t very instructive. It tells that there’s danger ahead but not how to avoid it. It would be better to say, “Be careful with that vase. It would be good to hold it with two hands.” This provides a pause in the action that gives a child time to reconsider the possible outcomes but also suggests a way to avoid disaster.
Third, “be careful!” limits a child’s actions. An active child is a learning child but “be careful!” cuts off learning. When our warning makes a child stop and wait for a grownup to do things for her, or makes a child stop and not try at all, then our warning keeps a child, not just safe, but little. Competence and confidence come from doing things. We have to let kids do.