In the pediatrician's advice I link to in this subthread, there's a "Common mistakes to be aware of at all ages" section, and about that:

"Goat's milk contains very little folate and its use as a primary calorie source causes megaloblastic anemia."

Cows milk over 32 oz/day will cause iron deficiency, and "In children this is a hypercoagulable state and can cause cerebral venous sinus thrombosis. I have see this."

Substitutes for human milk designed for human consumption turn out not to be trivial.