Berserk's problems start when Golden Age ends. Guts loses his personality and becomes a textbook stoic. Griffith, who was a solid anti-hero, is removed and replaced by an ideology on a stick. The cast becomes almost as lobotmised as Casca.

Golden Age is the only good part because there are emotional and physical stakes. There are nonsensical moments where Guts slaughters hundreds of enemies, but the stakes are there since he is just a strong man, not an overpowered magic knight with witches to solve conflicts.

Griffith is a character inconsistent with his ideology, which is both good and bad. It's good because it makes for an interesting internal conflict, but it's bad because it becomes a driving factor in why the Eclipse takes place. That means everyone dies because Griffith doesn't understand himself very well. Not the most intriguing development for a dark action show. Perhaps it would fit in a school melodrama instead.

For all its good parts and improvements upon the manga, of which you mentioned (plus the removal of most supernatural elements to make the ones that remained more special), it stills has significant flaws. Adon is the worst character in the show and drags many episodes down to his unintelligent level. There was a moment in the battle for Doldrey where Zodd gave Guts an extra sword, which was an asspull and cheapened the outcome of the fight.

Berserk is a decent show, but it has many flaws. It is nowhere near the masterpiece that many fans would like to claim, but it's worth a watch since no other dark fantasy can come close.