Normal situations a POD/TOD is (IMHO) very easy and simple.
However, in this joint account issue, I can see some complications and there are probably lots I don't see.
Couple has joint account and die in plane/car crash.
Husband's Will says give all to charity.
Wife's Will says give all to nephew
TOD/POD beneficiary statement says give to Couples Child.
Where there is survivorship in the Joint account agreement.
Did Husband die after Wife by 2/5/30 minutes : Joint account is therefore Husbands and all the $$ goes to Charity.
Did Wife die after Husband by 2/5/30 minutes : Joint account is therefore Wife's and all $$ goes to Nephew.
Both died at the same instant: Joint account POD's to the Couples Child.
Imagine the legal quagmire fighting over who died when