Given the overall constraint of condensing two full length (and wordy) novels into four hours of television I really believe that the series was a magnificent production.
It suffered only two problems, first the condensing mentioned above and second that there are no likeable characters in the story.
All the characters are insane, remember that Gormenghast is a society that hasn't changed for 1500 years (76 Earls), and is so introverted that insanity is the norm. I think the series captured that superbly. I rate it a success for the entire production team / cast.