By default, role='alert'
inherits aria-atomic
and aria-live
with an assertive
value, so should behave similarly from a user's perspective. However, not all user agents implement specifications in the same way.
The w3c recommends that you use an appropriate role when available, in preference to a generic aria-live region. This comes with the caveat that user agents have historically shown inconsistent behaviors between the two.
Testing against HTML5 Accessibility's alert test page demonstrates that ChromeVox 1.27.0 supports the technique you showed, while VoiceOver on OS X 10.8.4 with Safari does not.