These are objectively worse than just keeping your daggers in your belt, where you can draw/sheathe them without using a bonus action.

I think if it requires a bonus action it should also provide a beneficial effect, in my mind something similar to the Hidden Blade skill trick from Complete Scoundrel in 3.5e.  Rather than flat-footed I'd grant advantage on your next attack before the end of your turn once per dagger per combat if the target hasn't succeeded the Investigation check or seen the given dagger extended yet (for instance if you had one out then retracted it in plain sight of the enemy no advantage is granted when taking it back out).  It would allow you to get in extra sneak attack (or two if you spread them over multiple turns) when you don't have any other way to trigger it without needing something like the Swashbuckler's Rakish Audacity.  In my opinion this adds mechanically useful assassin flavor while keeping it from being overly abused.  It also rewards creative shenanigans if you combine dropping a held weapon/item as a free action (per sage advice) with the bonus action like "oops, I dropped my weapon and now I'm unarmed, just kidding".  At the risk of over complicating things it could also include the ability to extend one as part of the reaction for the Defensive Duelist feat, Battle Master's Riposte, etc granting advantage on your first attack with that dagger on that target before the end of your next turn.  Just my initial thoughts on giving it some more tangible utility, love the concept regardless!

That depends on the DM. Mine has ruled that drawing a weapon from your belt (except for the first weapon at the beginning of initiative) takes an action. Because of that, with these, you could use your bonus action and action to draw and attack in the same round. 

I would add:  When attacking with the Bracers of the Assassin, You have advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet.