That is fine. You can also simply say:
It is more complicated than I thought.
To add emphasis, you could say what you have:
It is much more complicated than I thought.
Here are some examples of the usage of the word, much.
How much does this cost?
Four million dollars a day is too much!
Much of what we see here is subject to interpretation.
In the last example above, you could instead say:
Most of what we see here is subject to interpretation.
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