In UI Designer if an input is for a number a user can input alphanumeric text. $form.$invalid is then triggered and the submit button can be disabled.
Nevertheless there is no error message associated with this input / error which leads users to seek for a forgotten mandatory input field.
It would be nice if an error message was displayed next to the input. Another nice addition (wishlist mode) would be a "regexp validation" for complex user cases (I know I can write a custom widget for this but I feel it would benefit the whole community)