-ing form with preparatory it

It's not a matter of which adjectives can figure in extraposition. Generally, any adjective that can function as a predicative complement in the non-extraposed version is possible.

In general, NPs cannot be extraposed, and the fact that gerund-participials are more like NPs than infinitivals are is seen in the fact that they can invert with the subject in interrogatives, and this greater likeness to NPs is reflected in their being less readily extraposed than infinitivals or finite clauses.

I haven’t seen a detailed attempt to say when gerund-participials are OK in extraposition, but certainly some cases sound fine. But I remember that in early work in generative grammar it was said simply that extraposition could not apply to gerund-participials.