Example, a multi-select, see https://jsfiddle.net/z0Ltxh47/1/ :
Usability is bad because I can't select multiple non-consecutive items by using just my mouse. It's very narrow by default, and doesn't allow me to resize.
API: Your typical HTMLInputElement.value doesn't give me all the selected values. I need to map the .selectedValues to their values, or innerText if no value exist. Horrific.
Accesibility: At least in Firefox, using ctrl+arrows to be able to focus on elements to select them do not render any focus targets. In Chrome, the contrast is not there.
Don't even get me even started with the date/time-picker.