The problem with relying on "security through obscurity" for developing your initial CAD models is that you won't be able to patent these designs. You're making a public disclosure about your concept before you file, so if you intend to protect your hardware concepts through patent protection you'd be better off using something like Autodesk Inventor Fusion 360 (which is free for hobbyists, students, and even startups making less than $100k/year, if I remember correctly). I think OnShape made a big mistake by removing the ability to keep any of your designs private without paying.

That being said, if you're working in the open source hardware space or just don't plan on patenting your work, OnShape could be a good option.