The problem with the checkbox is that it is not an HTML checkbox - ie <input type="checkbox">. Instead it is a md-checkbox, which I believe is from the Angular Material library. As a result, Watir's checkbox method will not see it.
You will need to treat the md-checkbox as a generic element and use the click method to set/clear it:
browser.element(class: 'ng-pristine ng-valid md-default-theme-ng-touched').click
The class attribute does not look unique. You might want to use one of the other attributes instead. For example:
browser.element(aria_label: 'Remove').click