Healing in ShB leveling dungeons can certainly test your ability to play a healer but it is in no way very difficult unless you plan on carrying the whole run. If you're DPSing and healing in leveling gear, it's mainly difficult if tanks cannot rotate their mitigations properly and if the DPS aren't using their DPS skills properly because damage from enemies is a lot higher and the HP values increased in respect to the healing skills' potencies so healers can't cover and heal all the mistakes instantly if the players make very punishing errors. With big pulls, you will have to GCD heal no matter what even if the party plays well. Essentially, ShB leveling dungeons starts expecting a level of competency on all players on knowing their job and learning the mechanics of the dungeon - which should be the norm. A healer at ShB dungeons can no longer be babysitting the team and covering for all mistakes as it once did or you'd just run out of resources really quickly.