>>238693911First things first, being an exceptional artist doesn't make you a good mangaka.
Miura is praised because he was both. Panel work, character design, art style, setting , world building, story, dialogue etc. all have to come together to produce a high quality Manga.
Miura excelled at many of those disciplines, his pure technical skill of drawing is probably his most highly regarded of those.
The manga itself has its ups and downs, most people consider everything up to and shortly after tower of conviction to be the series high point, while both the first few and last few chapters have been debated alot.
Overall it's the fact that there actually ARE 364 chapters of Berserk where the aforementioned qualities are met to a very high degree.