End ciy dungeos are very rare and it can be very difficult to find one, especially in survival mode. However, if you find one, you will seal it in less than 5 minutes. I think shulkers have very few health when opened, and they are always opened. it would be better to open just to shoot a projectile and then close instantly. it would be also good that they had some drops, or that they culd be tammed, as the dungeons don't have something the exlusive, that you can't find it elswhere, and i'm not enforced to go and find one