FMT prescribed by vet for chronic soft stool

I took my dog to the vet 3 weeks ago because he's had chronic soft stools for over a year. That vet suggested the most common cause is food allergy and to switch him to a limited ingredient diet and if that didn't work then to try a prescription diet.

I just took him back to the vet today for vaccinations and saw a different vet, who said that limited ingredient diet was likely not going to work and that I should instead try fecal microbiota transplantation and gave me a source on where to purchase pills for this. I'm not so sure about this as it seems like an experimental treatment, but she seemed to think it was a good thing to try as a first line option. Is this common?