I really like idea of the shield ultimate but they really need to add CC immunity just like regular blocking. In my opinion adding CC immunity would make this ultimate what it really should be which is an actual free block.
Honestly, it's in the description zos is straight up lying in the tooltips. Even if it worked as described it's more of a pvp skill, bolstering darkness just can't be beat.
There's numerous threads on this and I use this ult as a bit of a safety. It does have CC immunity attached to it like normal blocking, however it seems like some abilities are just a bit broken on it depending on the boss, etc.