The most compelling aspects of the books is that they actively try to remove the anachronisms (wooden hill forts, ponies and spearmen instead of stone castles, warhorses and swords), and they look up at the legendary characters from the viewpoint of Derfel. Legendary characters sweep into Derfel's life, have impact, then leave. They keep their mystique because of the viewpoint we have of them. The show desperately wants to shift this viewpoint to Arthur and make it his story, which rather misses the point.
Straight away we start with the king in a stone castle and Merlin with some huts in a forest because that is the stereotypical settings for a High King and a Druid. Gone are the hill forts that gave the books an iconic setting, instead this adaptation just feels like churned out rubbish; wannabe Game of Thrones, Uthred or LotR.
That the characters have all had their personalities put through the wash is the final nail for this show. The worst examples are that Merlin isn't chaotic, Nimue has lost her bite and Morgan is bland. They've lost the very traits that drive their character arcs.