If you stop paying, the designs will remain intact, stay private and you will be able to export them freely. If you want to keep editing the designs, they will have to be made public at that point. (Or you can start paying again. ;))
I would never use this for professional work. Hardware lifecycles are often measured in decades. It is common to archive the complete collection of design software so you can come back in a few years to make an improvement or add a feature. Your career cannot afford to tie everything to a cloud vendor that may decide to leave you high and dry.
I only see a future for this product if they license the software for installation on private servers.