Question on Headless Adaptive Form

We are looking to utilize the headless Adaptive form to display the form on an external React SPA. As i understand JSON representation of the form is targeted at /content so it includes the validations, patterns, field limits etc that we configure at component level and that stay under the /content node. 

But in case of complex requirements, where i need to write my own JS functions, this wont work as client libs will never be sent back through the API. Are there any alternatives to do this ? how flexible is the rule editor in handling a little to medium complex validations. Example, an API call that needs to be made on field select, SSN masking, Prevent future date from being picked etc.