These dramas share the same production team as well as a basis in the novels of Minato Kanae. Each deals with overlapping themes in a different way, though their atmospheres are similar.
Wakamono Tachi bears a strong resemblance to Under One Roof to an astounding degree. There might be more siblings in Under One Roof, but those in Wakamono Tachi share the same issues as many of them (albeit mixed around). The eldest brother in each is a hard-working, lovable goofball who talks loud and fast. There's a lot of bickering at the table, a young sister having an affair with an older man at work, etc. But aside from these and other similarities, both are warm family dramas with a lot of heart and a good message. Plus, they each have "the cast of the decade," so to speak.
Both are historical dramas based on true stories, centering on passionate individuals who worked hard to pioneer in the scientific field. The characters must combat public opinion, harsh terrain, and freezing weather to reach their goals.