In early implementations, only values that have been set using the DOM property can be read using the DOM property. This should not be a problem as implementation improves.
Not all ARIA properties will be implemented, since the HTML specification is still defining how properties using ID references will be implemented. Examples of ARIA attributes that will not be implemented in ARIA 1.2 include:
.ariaLabelledby: property for aria-labelledby attribute.
.ariaControls: property for aria-controls attribute.
.ariaOwns: property for aria-owns attribute.
Confusion on string versus boolean values being returned for some states with boolean values (e.g. aria-checked).