Atomic.io never really caught on, I feel like... but I've found it to be a comparable alternative to Principal, and for everything I've used it for I've been able to get away with the free plan. It allows for variables and form fields, complex interactions and components... I usually have specific tools to address the spectrum of complexity that I need to communicate in a specific project. Sometimes that's wireframes, some times it's simply high fidelity mockups, then low fidelity prototypes, more complex interactions, to high fidelity prototypes to fully functional prototypes that just aren't hooked up on the back end... following that same flow, my tools would be Invision Freehand, Sketch, Invision, Principal, Atomic, Webflow. Beyond that, I'm handing it off to development...
Atomic.io never really caught on, I feel like... but I've found it to be a comparable alternative to Principal, and for everything I've used it for I've been able to get away with the free plan. It allows for variables and form fields, complex interactions and components... I usually have specific tools to address the spectrum of complexity that I need to communicate in a specific project. Sometimes that's wireframes, some times it's simply high fidelity mockups, then low fidelity prototypes, more complex interactions, to high fidelity prototypes to fully functional prototypes that just aren't hooked up on the back end... following that same flow, my tools would be Invision Freehand, Sketch, Invision, Principal, Atomic, Webflow. Beyond that, I'm handing it off to development...