No, problems with the magnet and the pickup would only cause a loss of "clicks". That is, problems here would only underestimate the speed.
That's right, and consider my experience as well.
For years I've used rollers as trainers and had wired computers for speed/odo functions. Over the years, I discovered
I could "wear out" the magnetic reed switch in the pickups!
That's right. After several years of use they would become "sticky" and start sending erroneous info to the computer. They would work perfectly at 20-30mph, but during sprint efforts and 35mph+ they would "lose it" and drop out to zero intermittently.
Radio interference is always a concern with low power consumer gadgets. You've been "gotcha-ed"