What made the Hammer Horror movies great was that they were the first to bring more adult themes to horror movies. What their Frankenstein series got right is that the real monster was the mad doctor, not the pathetic creatures he created.
In addition to their work for Hammer, Cushing and Lee also did a lot of movies for other studios, most notably Amicus. Often lower quality in terms of story and production values, but you had a few gems in there.
The biggest missed opportunity with Hammer was their attempt to do a Sherlock Holmes series with Cushing as Holmes. While Cushing was almost as perfect for the role as Basil Rathebone, the story they picked, Hound of the Baskervilles, underutilized him and the series never really picked up.