A Custom may have (no guarantees from what I've seen posted here recently) better fit and finish.
Depending on who makes it, he or she can make it exactly as you want.
Customs always cost more, and do not necessarily use a "better" blade steel or materials than a "similar" production knife.
Seriously: Does a custom cut/slice better than a production knife?
Can a custom do anything that a production knife can't?
(aside from selling for thousands of dollars, and "impressing" another knife nut, perhaps)
The warranty on a custom ends when the maker dies or retires, which may or may not be before the original owner does, and the knife is out of warranty, anyway.
The warranty on a production knife outlives the person(s) who assembled it.
(though most production knives have a "Life Time" warranty that ends when the original owner dies, or sells, or gives away the knife, or when the company shots down, which ever occurs first.)
I'll stick with a production knife. they do everything I need a knife to do, and are "affordable" (to some extent and depending on maker/brand) on my fixed income.
I'll use the money "saved" that a custom would cost for practical things like more knives, rent, groceries, Mountain Dew, clothing, perhaps a vehicle ... maybe a hunting and fishing license and the required gear ...