From the description of the flex plugin at http://www.grails.org/Flex+Plugin
"This plugin was created as an experiment to prove that it's possible to communicate from Flex to Grails services without any configuration in Flex. There
is no plan to add functionality or whatsoever. It's not sure if the plugin works in production mode as it is only tested in development mode. This plugin
can be seen as an example on how to integrate Flex and Grails with Convention over Configuration in mind."
There is no scaffolding generation for flex, see this bug
Another disadvantage of Flex is its lack of accessibility for screen reader users with the default configuration. There is some work being done to address this but it isn't publically available yet.
I don't have experience with DOJO but according to this link if you use the standard Grails Ajax tags you should be able to switch libraries with out to much trouble and won't be tied to Dojo. For fancy UI stuff such as tabbed interfaces and data tables I've found GrailsUI works well for me. A demo project showing its features can be found at http://code.google.com/p/guidemo/ although you'll have to check the code out to run it.