taranoire:

Loki stabbing Thor and Thor using the obedience disc on Loki is like…..the equivalent of pulling each other’s hair. They’re not hurt that easily, and especially for the latter, it’s been well established that Thor would never ever actually seriously hurt him. He did it to immobilize him temporarily so that he didn’t fuck up Thor’s plan to, you know, save Asgard. 

This discussion has also been had ad nauseam. Have you seen the Ragnarok shooting script? If not, it’s here. The script describes Loki as “WRITHING in pain” and “convulsing in agony.” So even if the disk can’t do too much damage to him, it is still causing considerable pain. That is even the goal of much torture these days: to cause unbearable pain without causing lasting damage.

@foundlingmother has given this thoughtful response to exactly the argument you’re making now (with some additional comments from me).

The one thing I would add is that it has been well established that Thor would never seriously hurt Loki. Thor’s characterization in Ragnarok is so radically different from his characterization in previous movies that I don’t even consider him the same person; accordingly, I’ve started calling him Thor* (following a notational convention in academic philosophy). From the fact that Thor* left Loki defenseless on a planet full of people who would melt him without a second thought, I cannot conclude that Thor* would never seriously hurt him.