I just googled the issues related to mortgage downpayment gifts, and it looks like you're correct. This is the best summary of issues I've found and it doesn't mention anything about being higher risk in and of itself:
From that page, the bank wants to make sure it's not a loan or a side-payment from one of the parties to the transaction.
Still, I'd be really, really surprised if there weren't a correlation between "fraction of DP as gift" and "default rate", but I don't have anything concrete to cite ATM.
https://www.accunet.com/buying-a-home/can-my-down-payment-co...
From that page, the bank wants to make sure it's not a loan or a side-payment from one of the parties to the transaction.
Still, I'd be really, really surprised if there weren't a correlation between "fraction of DP as gift" and "default rate", but I don't have anything concrete to cite ATM.