There must be a limit though. Surely a light saber is not more safe than a dull knife.

If you’re used to sharp knives then it’s probably safer than applying more pressure which could yield in surprising ways. Getting used to it is the dangerous part, especially if not forewarned or the user doesn’t heed the warning.

Lightsaber would be different because doesn’t have a blade to guide.

Not really. The dangerous aspect of a knife is when it moves unexpectedly. The sharper it is, the harder it is to create a scenario where it jumps on something particularly dense, like when a saw hits a knot in a tree.

If you’re pushing down with hard force, it basically doesn’t matter if the knife is sharp anymore, it’ll just chop your finger off. However, with an extremely fine cut, it will be much easier to reattach, as the edges will match up well. With a dull knife, you’re not slicing, you’re more tearing your way through something.

It's not my impression that if you make a knife very very sharp it turns in to a lightsaber.

Actually, a light saber is 100% safe since it doesn't exist.

Imagine how dangerous a dull lightsaber would be