Anvilicious: According to this article, the franchise was meant for kids who grew up from late 70's to 80's, showing them the struggle that their parent's and grandparents went thorugh after World War II.
Archive Panic: And how! The series has 31 entries and each of their episodes are around 25 minutes. Then, there are A Dog of Flanders and 3000 Leagues in Search of Mother movies.
Fandom Berserk Button: Considering how it, to this day, are classics and part of history of Japanese animation, criticizing the franchise online is a terrible idea. Absolutely do not make fun about its storytelling.
The franchise is this as an introduction of Japanese animation in general.
It also introduced some obscure or not so well-known novels such as A Dog of Flanders, Southern Rainbow and Romeo's Blue Skies.
Germans Love David Hasselhoff: Look at the Wikipedia page for at least 60 percent of these shows in Italian, French or Arabic and they'll likely be twice as long as the language of the page you were previously reading.
Just Here for Godzilla: The reason why many people watch Heidi, Dog of Flanders, 3000 Leagues in Search of Mother, Rascal the Raccoon and Anne of Green Gables is that Hayao Miyazaki and Isao Takahata worked on those shows.
Slow-Paced Beginning: The franchise suffers from this for those people who are not interested in settings, additional back stories and characters who are not originally appeared in the novels. Each of the anime takes around few episodes to see new characters and to pick up the pacing of the plot. This is even lampshaded in the episode previews of Les MisÚrables: Shōjo Cosette.
Tough Act to Follow: The rest of the entries after Anne of Green Gables never managed to receive the same level of acclaim.
Vindicated by History: Entries like Romeo's Blue Skies and Tico of the Seven Seas received poor ratings on their first run in Japan, but nowadays they are chosen as one of the best WMT shows according to a poll by Nippon Animation. Especially the former, who is placed in second beating Anne of Green Gables.