Simple problems are easy to model with the UML. Hard problems will become easy to model as you become fluent in the UML.
Reading about the UML is one thing, but its only through using it that you gain experience of it and become proficient in it. Depending on your background, there are different ways to approach using the UML for the first time. As you gain more experience, you will come to understand and appreciate its more subtle parts.
If you can think it, the UML can model it.