Anything that isn’t conductive, such as wood, plastics, rubber, etc.
That isn’t a bad thing, because we can build fixtures, holding devices, etc. out of those materials when using induction. The limitations of induction are there, such as heating a large piece, say the size of dining room table, and it may be better to use resistance heating with that type of application. So it is application specific. With induction, if it fits, it’s probably the most efficient way to heat parts. But there are applications where we do recommend using a different type of heating. But it depends on the process, how the parts are handled, etc.