I think it is slightly underrated, but only slightly.
First, let me stress that it is flawed and does have glaring weaknesses, especially in panel arrangement, pacing, and sequential storytelling. So, I don't believe it should be held as a piece of classic literature. However, I don't believe the story itself gets as much credit as it deserves, and the manga does tell a much more consistent story than the anime, preferring soft resets after story arcs with elements of ones carrying over to the next. I might not like many of the developments in the manga, but overall, plot-wise it is more coherent and connected...if you can tell what is going on (and some of that confusion is due to the nature of manga itself).
To many fans, the old anime is the only version of Sailor Moon that matters, even though a lot of the fanon that applies to the anime is rooted in the manga. If you were to look at the old anime without any knowledge of the manga, there are a lot of developments that come out of nowhere and don't make sense. By contrast some developments are foreshadowed more in the manga. Plus, Takeuchi did pull from a lot of deep mythological sources for the building of this fictional world.
I definitely prefer the old anime...but I have to acknowledge the writing is weaker than in the manga.