One of my friends is an emergency vet and she and I spoke about this. She also owns Leonbergers. There is a HUGE amount of controversy and conflicting information on pet foods in general.
One site will say X food its the best food EVAH! and another site will give it zero stars and accuse the company of trying to poison pets.
She's the one who told me "You're not going to go really wrong with a higher-end kibble from one of the MAJOR players in the industry. That's was when I was looking at pulling Seren off her organic raw meat/veg diet because I got tired of her refusing/wasting more than half of what I put down for her. It was FAR too expensive for her to be pulling that *expletive deleted*it.
I ended up going with Iams Lamb and Rice, but then switched over to Purina ProPlan when I ran out of Iams when I was staying with Castle Key. He was feeding the ProPlan at the time so I gloamed some from him and Seren absolutely inhaled it. She loved it, and she still likes it. The one mistake I did make was than I moved her to Chicken ProPlan and that turned out to be a huge mistake because I'm pretty sure it caused allergy issues that resulted in ear infections that I'm still fighting. Things are better since I switched her to the Lamb and Rice ProPlan, but it still has poultry by products in it, so I'm thinking of trying to switch her to one of the Lamb and Oatmeal sensitive stomach and skin formula as it pulls all of its glucosomine source from fish boyproducts instead of poultry byproducts.