Any extension you write at this point might need work to make it compatible with the stable release. There is no doubt about that. To give you an example, all database migrations in extensions written for beta 4 and prior will need to be rewritten to work on beta 5. @Franz will be announcing this in dev soon.
To what extent your extension won't be compatible depends on what you implement and what features of Flarum core you use. As Flarum is not yet released in a stable form, decisions might be made that break extensions.
