Healing arrows I found worked reasonably well when my thrall was fighting a boss crocodile. I shot the arrow at the head of the crocodile and the healing cloud was around my thrall when they fought so he healed. I didn’t let him lose too much health (about 1/5 his health bar on a 6500hp thrall) before I used the arrow and he was back at full health before the cloud disappeared.
Part of the issue with thrall healing is that they do not regain health points over time if you are not logged in even on persistent multiplayer servers.
When you are logged in and close to them you can observe that they regain health at about 100 points every 53 seconds (yes I timed it). But if you log out and back in, then the thrall will still have the same low health.
I’ve made a bug report about the lack of time-catchup regeneration a while back: