continuous motion with a line scanner is problematic when there are splices. The result is a kind of warping at the splice. This is a common problem with all the line-scan based machines like the Spirit, Shadow, Scanity, GoldenEye. In fact, it’s so bad in the GoldenEye that DigitalVision includes a copy of their restoration tools with a plugin that fixes splice bumps in software. Kind of crazy. And as someone mentioned above, no possibility of multiple exposures per frame.
The Northlight used a different approach: the film is moved into a gate and held firmly in position, then the whole gate is moved past a line sensor. This eliminates the splice bumping that you get with continuous motion, but it’s a much slower process. That particular scanner took seconds per frame, even the faster Northlight 2 model. Great scanner, beautiful piece of hardware, but painfully slow. Modern hardware could probably be made to run faster but there’s a lot of extra engineering that goes into that.
A camera with a rolling shutter kind of works like this though, it’s just all done in the camera. We use one on Sasquatch (film held still then separate red, green, blue images are taken of that frame and processed in software into a color image).