An ortho attending gave as all the following document that I think is a decent summary. I think he would like all ER residents to know the material so that he and his fellow orthopods get less calls and less, "what do you means?" on the phone.
My local orthopods don't share his loathing for orthoglass. They themselves use it in the department sometimes. I asked them specifically about it and they had never heard that orthoglass is less ideal than plaster. The reason that the author of this document thinks orthoglass is of the devil is that he thinks it is prone to causing pressure necrosis over joints and seems. I think it isn't as much of a problem as he says, but certainly, check with your local orthopods to see what they prefer.
Personally, having spent my entire residency being forced by ortho to put on plaster casts, orthoglass is like a breath of fresh air and makes splints much easier, and more importantly, faster to put on in a busy ER.
Also, you need to use a little common sense. If you immobilize the joint on either side of the break, you are probably going to be splinting it adequately.