Look at the film.

They don't present any rules AT ALL about the Ion Cannon.

What we know:

The Ion cannon can apparently disable, at least temporarily, a Star Destroyer.
It is not used against a ground assault.
It is not used frequently.
We only ever see the one.

It's not up to the viewer to invent rules about fictional technology, and then wonder why the film doesn't follow them. We can extrapolate from what we know, and this can be fun, but if what we extrapolate contradicts what's in the film, the film itself does not become 'wrong.'