Mewing advances the chin.  Getting into class 3 is not good.  It's meant to take people out of class 2 to class 1.  Many people don't understand this and are in the same boat.  Expanding your palate isn't going to help at this point.  Please send photos!  Also... age?

Since I got no responses I'm just going to post here what I found online after a little digging. Not much tho so don't take my word for it,im no expert. 

Contacted Dr won moon on the subject and he did tell me Damon braces can  promote appositional bone deposition more than regular braces even tho it's a lot to do with the patient and the buccal bone. I think it's something that can work very well for the people that only need a few mm of expansion and have enough buccal bone to keep their teeth centered.

Damon also places a lot less force on the teeth.

Damon phylosofy practioners also are a lot more focused on expansion orthodontics with no extractions then people who swear by regular braces and think extractions are the only way to go. So it's safe to say that, comparing them to regular orthodontists, they re professionals who definetely have an airway and facial approach in mind. 

Damon smiles assymetries usually also happen  from treating their patients always on the same side. Probs could be dealt with just fine if you let the doctor know about it.

Proven to help a lot with tmj, tho not completely fixing it. again tmj has a lot to do with a small uneven palate, teeth biting higher or lower on different sides, crossbites and clenching more on one side then the other etc.. Again, if the expansion you need isnt a lot, I'd say they re viable. 

Bottom line is, in my opinion, 

If you need just a bit of expansion I think they re a very viable solution tho not on the cheap side. but you get straight teeth and function.

Portuguese here. 

But anyways, you have to go to another place that will help you.