There are many challenges to working with 3D printing. Finding a good 3D CAD program is just one of them. I like using SketchUp Make and I can get things done fairly fast now that I'm up to speed on how to use it.
However, the biggest challenge, in my opinion, is how to design your 3D object so that it will actually produce a good 3D print. If you've not worked with 3D printing before, then you probably will have no concept of what this means. It means that just because you can design it and create it in your 3D CAD program, doesn't mean you can print it.
Much of the time, you'll have to re-design your part so you can make a good 3D print of it. Things that stick out and over hang won't print well at all. If you must have an over hanging part, you'll learn to add 'supporting' legs in your CAD drawing so your printer will build up the support filament needed before it starts to lay down the over hanging part itself. Then, afterwards, you'll have to manually cut away this supporting material and clean up your part.
When you look at your finished CAD drawing, you have to decide which side will lay down on the hot bed? It's not always obvious.
With experience, you'll learn and see in your CAD drawings what will work, what might work and what won't work for sure.