Not all of them are that UP.
Phase shield and wings aren't too bad to give up when they are about to expire...
same with boneshield.
The grav one is definitely worth it imo. It would make "its a trap" doable as a duo... that could get ugly (put this + grav pull on a titan and watch the enemy weep)
The 
one gives gaurd, which could be quite devastating if played correctly (for instance, if a heavy hitter gets AM'ed you now have a powerful healer. Also works well to let a heavy hitter stay occupied in case of SoSac)
Still, I see bogtro's point some of these shields are quite expensive. I'd say the creature should gain a stat bump (hp, and maybe att) as well as the ability. Maybe make the extra hp relative to the cost of the shield.
Remember that this card initial purpose wasn't to make an improved "flying weapon" but to give an use to surplus of shields (example when oracle give you a shield card and don't know what to do with them)
Yeah, some of the shields are expensive, but the abilities can be tweaked later. Also my first idea was turning the shield into a permanent (so you had two) but then I noticed that wasn't good (some shields are better than others that way) and also favored stalls which is one of the things I didn't really wanted to do.
Dusk gives burrow instead of invisibility because it was the first thing it come to my mind thinking on the pesky devourer . An option would be placing the creature underground by using it on a dusk shroud, but a "burrowed" lava elemental, or Otyugh could be rather nasty, since there isn't any card that can "unburrow them"
What does invisibility?
