What is jarring is that when 3D models are used in 2D animation, the deformations that occur in 3D are completely different looking from the way 2D deformations are animated, so even unifying the framerate doesn't really help.
Yeah. I can see the logic in using 3D for rigid objects like vehicles and buildings, but it is a fundamentally different-looking style of animation.
I remember that when one of Disney's Roger Rabbit shorts way back when used 3D computer animation to create a roller coaster, what they did was to print out the CGI, have the animators hand-trace it, and animate the resultant cels normally, so that it matched the hand-drawn look of the rest. Which clearly didn't catch on.
My favorite pre-CGI technique for animating 3D objects was the one used in Filmation's
Flash Gordon and the Taarna sequence in the movie
Heavy Metal -- building physical miniatures painted white with black edges and detail lines, photographing them, printing out the photos on animation cels, and cleaning up the lines by hand.
I was always very impressed by the animation of 3D objects like vehicles and buildings with shifting perspective in
Akira, but I'm not sure whether it was done by any means other than just drawing very, very meticulously. It certainly looked hand-drawn, but very accurate, so maybe it was traced from models.
Except for Futurama. The 3D Planet Express ship just plain works.
Yeah, that show did a really good job integrating 2D and 3D animation. I think most of the "sets" were 3D environments.