Ultimate Venom was a story that had a huge amount of potential, and the setup and first half or so of it was an amazing new approach to the symbiote saga --- but as it went on I think the third act of it just kind of fell flat.
Ultimate Eddie Brock, while very similar in character to the 616 version, never seemed to have any real sense of motivation. While 616 Brock's motivation is admittedly somewhat contrived and even has had to be reworked several times to make more sense, its still always had this sense of
intensity, that the Ultimate version just lacked. It didn't help that the actual Venom character was basically a silent, human devouring cypher, that lacked the menace of his 616 counterpart (At least in the early appearances.... yes, I'm pretending Lethal Protecter didn't exist

).
I still liked the storyarc in Ultimate a lot, but I think the animated series approach that the movie seems to be going for works far better as a much more complete, developed storyline.