… not necessarily. The usefulness of PWM-based LED-controllers is related to the frequency they are operating on. With a sufficiently high frequency, you won’t run into any problems. I simply do not know whether this is an issue or not.
Again, maybe someone else on the forum has experience on the usefulness of PWM-based LED-controllers in scanning applications.
Here’s an example where you can see flicker in stage lighting caused by PWM, and here’s a discussion about color shifts under PWM. Whether this has any relevance with respect to film scanning, I can not tell you. You probably just have to try it out.