Why do you not like Ben Barnes as The Darkling?

For two main reasons:

It was pure fan service which I hate. Like the same way you see Timothée Chalamet being casted everywhere or being fancasted for every potential role, they put Ben on this role to attract audiences. It was pure marketing and it was obvious that they never thought: "Does that role suit him? Does he look like the Darkling? Can his acting persuade others that he is the Darkling?" (I say "no" to every question)

The role didn't suit him for me. He physically didn't look anything like him and his demeanor or acting reminded me nothing like him.

And generally I didn't like the show version because:

He was too emotional. Literally he was getting teared up every five minutes. When the creators said "We want to make him human" they meant "We'll only make him too human" and that kind of acting took away the gravity from important scenes (like the name reveal scene or his death). For the audience it was just another emotional scene but for the readers those scenes in the books were special and utterly heartbreaking because they could glimpse his human side that apparently still existed inside of him.

He was too much of a simp. Book!Darkling always had pride and even though indirectly he tried to beg for her help and understanding, he never threw himself at her feet (the only times he kinda did was in his emotionally vulnerable moments which were rare but precious). Show!Darkling was well... Wasted but not with the cutie meaning of the word. It became his whole personality.

(actually the name reveal scene doesn't even exist in the series)