I can tell you what Sam means about not fully understanding Venom.
I'm a film director. I just recently finished a horror film called Home Sweet Home. We've had 80% of the film in the can and edited... until this past Saturday, when we filmed the interrogation scenes of the movie with the detective. This was an entirely new aspect of the film. It completely changed the way the picture as a whole was, but in a good way.
This is much like Sam Raimi's predicament. I didn't really know how my story would come out before I filmed those scenes, because they were an entirely different element. Sam did direct the special effects shots, but he's not the one coming up with the CGI effects for Venom, and I bet the picture isn't fully edited either. So, Sam is waiting for everything to cut together and the full effects of Venom to be complete before seeing exactly what Venom is all about. And remember, he's talking about the Villian, not Eddie Brock. If he's able to combine the two into one, he'll see Venom for the way he trully is.
You know, you can't be sure of everything. Sometimes you have to trust that it will all come together in the end.