There are five types of Motion projects, four of which “sync” automatically with FCP, yes. Namely all but the regular Motion project since that can’t be imported into FCP directly either way.
So if you created e.g. a title, saved it, used it in FCP, reopened it (either via FCP or the Finder), changed it, and saved it again, those changes would be immediate in FCP. Merely any previously used instanced of said title, generator, whatever in any given project would need to be replaced by the (now) new master from its respective browser. Existing instances of a Motion element in a project/timeline are (fortunately) not affected by changes.