I used an amigo turnout as a stable rug last year with a full neck and there was no loss of mane, however the horse did have a particularly thick coarse tough mane. Not had to put the neck on yet this year.
I have bought two from ebay (can PM you the seller) they are very cheap but they are brilliant, never had a problem with mane rubbing and mine wears his 24/7.
fingers crossed my Premier Equine stable rug is working so far but there is time yet. I do sometimes think if your horse has the type of mane that is going to come out you just have to accept it if you want to use neck covers