Idiom for something that appears to be easy but is in fact hard

"surprisingly complex" could work well.

The word "complex" seems to reflect well on a course with rich content.

"surprisingly" signals the unexpected nature of the complexity.

If it were only a single simple question that was more complex you could simply say "tricky" however I do not believe that would apply well with the body of content in a course.

There is more to it than meets the eye is a phrase used in these circumstances.

The course is in fact far from easy.
The course is in fact anything but easy.

TFD(idioms):

far from
Not at all; anything but:
You are far from a failure.

American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

anything but
By no means; not at all:
I was anything but happy about going.

American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.