Disclosure: I co-own the business that does both custom development and Wordpress development.

While there are number of plug-ins to organise membership website for Wordpress, I don't think that you can build something like Clarity there.
Like others wrote in your case Wordpress can be a way to test the water, prove that there is a demand for your solution. You'll get something running fast albeit you have to go with what is available rather than develop it the way you see it.
Sooner rather than later you will overgrow the platform: the solution will be too complex, to slow, to hard to maintain and modify. This is a point to leave Wordpress and develop custom solution with Ruby or other technology.

Should you start from Wordpress or Ruby depends on many things, the most important in my opinion are;
- Where is your innovation? Is it on tech side or more on business side?
- Can available Wordpress plugins meet at least critical requirements to your MVP ?

Feel free to contact me for further discussion of your projects.