They also cannot be summoned while you are in combat or if the summoning character has not yet completed the associated introductory quest, even if the collectible has been unlocked by another character.
On the livestream they said something about companions keeping their progress account-wide between characters, only you have to unlock them with each of your characters by doing their intro quest. So if you have 18 characters, you'll have to do that same quest 18 times, but they shouldn't lose their progress and armor when you switch characters. At least that was how I understood it.Even worse: characters that have not done the introductory quests, cannot summon the companions. So they are not accountwide!