Billy Hargrove's trauma isn't comparable to anyone else in the show in my opinoin.
This is complicated to respond to. Billy received the most extreme physical abuse, yes. But the comparison of trauma isn't good, and can be harmful. It could potentially imply "my trauma means more than yours" or "I relate to this character, the character you relate to hasn't gone through enough to deserve trauma". It could also be as simple as a characters abuse shouldn't be ignored on how much it impacted the character.
A lot of people relate to different characters trauma. For example Jonathan. His response to his abuse, which from my memory was more mental/emotional than physical, is very real for some people. And implying that that trauma is less than, is harmful.
Another example is Mike Wheeler. His neglect from his parents was obvious. And childhood neglect can be seriously harmful. Implying that that trauma is less than, is harmful.
Or Will who was called slurs from such a young age. I'm a gay man and can't imagine how it would feel if that were to happen to me. I'd say I hid my homosexuality pretty well due to my homophobic environment. It was mentally damaging pretending to be someone that I wasn't. Implying that was less than is harmful.
Eleven went through things that people can relate to with how it left her mentally. Asocial and antisocial people could relate to her behaviors after the life she went through. Implying that is less than is harmful.
Trauma is trauma, it is never a competition.
If I misinterpreted this ask I deeply apologize, and if I came of as condescending I deeply apologize. My intent doesn't matter though, my impact does.
Tell me more if you would like. And please be considerate of other people's experiences if someone chooses to share theirs.