Both my dogs were on JWB for a few months, and although they didn't have a severe reaction like you've decribed they both had poos a bit too squishy for my liking, and Lucy (my cavalier who has allergies) was very flaky.
As far as i could figure out JWB only has "no
added artificial colours/flavours preservatives"
I've now got them both on Autarky (which is a huge amount cheaper!) and they couldn't be better.
Both really shiny, and Lucy's whole coat texture has changed, she's all soft and lovely now.
Autarky is also completely
free from artificial stuff.