The Jāṇussoṇi Sutta is about merit or benefit accruing to the receiver of the gift.
The Ratana Sutta is probably about merit or benefit accruing to the giver of the gift.
Is so, there appears to be no conflict.
In the Janussonin Sutta, the Buddha states that food offerings to the dead cannot reach them if they are reborn in hell, as animal, as human or as devas (heavanly beings). However, food offerings to the dead would reach them or other ancestors who are born as hungry ghosts or petas.
This is because a human or god does not need the gift, which is why the gift does not accrue to them. For example, a virtuous (human) parent gains merit from giving to its children. When a child gives a gift to its parents, this gift is only a trivial or amusing thing to the parent (rather than something that changes the parent's merit). Or when a rich person (a god) is given a gift, it does not change the rich person. For example, my rich friend returned home from overseas recently and when I visited him after many years I sensed I was not really appreciated by him because he has become very important & rich. But 15 years ago when he was having relationship problems & I helped him, he appreciated me because he was a hungry ghost then, wanting a successful relationship.
Similarly, for beings trapped in hell or animal mind, a gift does not accrue to them because of their chronic state. For example, when people are chronically depressed due to a deep loss (which is 'hell'), giving a gift does not help them. Similarly, an animal, such as a sexual predator, does not respond to gifts because they do not have the disposition for loving others. For example, I know a man who is a sexual predator, who is bitter & twisted. I helped him once for many hours set up & learn to use his new computer but he is never friendly when he sees me. He hates everyone. He is incapable of gratitude because his mind is animal mind, every morning & every afternoon on the same beat looking for tourist women to have sex with.
But for a hungry ghost, a person who is lonely or searching for love, a gift accrues to them because the hungry ghost is searching for love or gifts. The hungry ghost wants to be reborn in a human relationship & they respond very positively to receiving gifts.
In the same way, a family member who has not done enough merit to be human (enlightened) or a god & is also not an animal or hell being but is separated from their family & misses their family will accrue from a gift from their family. This is what the hungry ghost is here, namely, the needy & unenlightened family member who misses their family.
Its like my mother, who I see for one or two weeks per year. My giving gifts to her does not change the enormous merit she has made as a mother. Therefore, my gifts to her are essentially worthless, apart from a token gesture to show I think about her. However, my small token gesture is important to the 'hungry ghost' element in my mother; namely, her need to know her children think about her on her birthday, mothers day, Xmas, etc. Just a small gift or phone call is enough to keep this hungry ghost element in my mother satisfied. I am not required to give an enormous gift to my mother.
The above analysis is supported by the traditional depiction of the Wheel of Life (below), where the antidote, solution or practice for each realm given by the Buddha is depicted as follows:
humane - renunciation
godly - beauty
animal - morality
hungry ghost - generosity
hell - 'the book' - the Four Noble Truths