My multi-grafted mango with several varieties that I grafted last year have all grown to nearly seven gallon size in a even fashion. The San Felipe, Fernandin, Lemon Zest, and Angie all set and held fruit on the grafts from last year.
My three year old Lemon Zest has about 70 mangoes on it. My two year old Pineapple Pleasure has about a dozen.
I would think that nurseries would shy away from selling multi grafted trees for several reasons. Try to meet the customers demands on which varieties they would want, which of those would actually be compatible, growing out the grafted trees for sale (after grafting onto a healthy 15 gallon size at least), and just dealing with the general public. People just don't want to pay for the labor and work involved in getting that all together. I have advertised 15 grafted mango trees for $50 before and sold three in two weeks. I would think a 15 gallon custom multi grafted tree would require a retail price of about $125 per variety or more. That in a market place that people shy away at $15 grafted one gallon trees?