Junji Ito Collection might impress you more if you have not read the manga, people suggest. Let us try to understand why.
Junji Ito Collection (Junji Itô: Korekushon) was eagerly awaited by the fans, but when it came out, it turned out that the people who had not read the manga were giving it better reviews than the ones who had. In other words, the anime did not turn out to be a treat for the fans; it was better-received by non-fans who were just getting into Junji Ito.
That is clear from reading reviews on any website, such as IMDB, which gives the series a 6.6 score. With a total of 14 episodes, which cover various short stories by Junji Ito, this horror anthology is mostly considered mediocre by reviewers. The animation process did not improve anything about the manga, it seems, and in some cases, it might have taken away from it.
A rare reviewer who had not read the manga might consider the series to be very interesting, which might be attributed to never having been exposed to Junji Ito's work.
You see, the anime is very faithful to the manga, often following it panel-by-panel. As a result, the stories are there, the plot is unchanged, and it has already been critically acclaimed, so it is not surprising that a person who sees it for the first time would be impressed.
However, as many critics argue, animating the series was meant to add something to the manga. Manga is very limited in its abilities; there is, of course, the page turn method, which Ito employs in his manga. That means creating suspense by showing a character's reaction to what the reader is about to see, and when that page is turned, to unleash something horrific upon them.
Manga has its methods of instilling horror, especially when the manga artist is Junji Ito, who spends a lot of time on his horror panels, doing his best to make them seem believable, realistic. After all, the more real the horror on the page looks, the more fear it will cause. But there are also a lot of limitations.
Animation has way more tools. It can cause fear with moving pictures, and specifically animation has the benefit of being able to bring anything to life (unlike movies, for which the special effects are limited). The animation should have built upon Junji Ito's works and made something much scarier.
Instead, it is just the same panels as before with some animation that does not add to the scare and often loses horror points when the detail of Junji Ito's work is discarded in favor of much less detailed elements.
As a result, if you have not read the manga, at the very least, you get quite faithful retelling of Junji Ito's stories, which are quite interesting. If you have read the manga, you get a less scary version of the stories you already saw a great version of.