Obviously, spoilers ahead as there's no need to hold back when it merely fleshes out some of the established plot points of the original manga such as the circumstances surrounding Aiolos' escape and highlighting the conflict between Saga's two personalities. The latter is probably the highlight of the spinoff with it showing the guilt his true, good self feels very well. Perhaps the most surprising reveal of the story is that of Aiolos' Shadow Arrow technique, a unique restrictive move with specific conditions that's far more interesting than the typical rapid fire punch that was the Atomic Thunderbolt in his anime appearance or just him firing an arrow as is typical of a Sagittarius Saint (though not quite as impressive as something like his Infinite Break in Episode G).
Perhaps the two most controversial parts to casual fans of the series would be Aiolos not donning his cloth and Shura attempting to kill Athena, neither of which are issues for those who paid attention to the original manga. With the exception of those that have strong psychokinesis, cloths were put on manually in the manga rather than automatically and it was established depleted stamina and Cosmo could cause one to be unable to use their cloth efficiently, so it may have only hindered Aiolos. Those who complain about Shura's characterisation forget that his manga self and anime self are two completely different characters. Manga Shura made it evident he followed the same might makes right policy as Deathmask and Aphrodite rather than being a loyal follower of Athena (Aphrodite even citing him as being one amongst the three to have betrayed Athena and sided with Saga), so this is just Kurumada sticking true to what he showcased in the original manga rather than retconning Shura's character in the manner Episode G did. One piece of characterisation that is questionable though is that of Aphrodite being struck by Aiolos' kindness and sense of duty only for it to never come up again and him wholeheartedly betraying Athena. Sure, 13 years can change someone a lot, but the manga never really does anything to smooth over this development.
If there is one thing that stands out as a genuine flaw in the manga though, it's the portrayal of Aphrodite and Deathmask's age. They should be 9 and 10 respectively and were drawn with a childlike appearance during the events of Volume 13 detailing Saga and Aiolos' situation at the time, yet here they're drawn to look no younger than their mid teens. This is a pretty obvious inconsistency, though at least not one that affects the plot in a meaningful way.
As for the art, it's fine. Many people complain about Kurumada's style and its lack of change, but if it's not broke don't fix it. It may not have the detail in faces as Shingo Araki's take on the anime, but its unique nature makes it still far more worthwhile than the majority of modern works. I will say, however, that Kurumada's art here is less consistent than that of the original manga with some faces look very off (particularly Athena's in the epilogue) such as Deathmask's overly comedic portrayal during his confrontation with Aiolos.
Overall, Saint Seiya Episode Zero isn't anything worthy of praise, but is not bad as some may claim. It presents enough interesting aspects to boost it slightly above average and what flaws it has ultimately amount to nitpicks. A decent read for fans of the Saint Seiya manga wanting a little more detail on some events.
