In my experience, I've had a pretty significant blob of webby fungus growth in an RB lens; it cleaned up and hadn't etched the coating. I'd say it's always worth a try (and I got that lens for free when buying a 65mm - now I have a mint-looking 127 with no fungus).You're violating some laws of physics and chemistry here.
As I mentioned earlier, fungus excretes hydrofluoric acid, which etches glass.
If you catch it early enough you might clean it before it eats through the coating.
But those wouldn't be "totally spider-webbed".
- Leigh
I also have a cheap Canon 100mm FL lens - those go for thirty bucks or so - that's badly fungus etched and cleaning didn't help it. But F me, I've shot 4K video with it and it's like god's own diffusion filter - details are sharp, but there's a glow and softness that's just amazing. Between that and the 1960's color and contrast rendering, it's a glorious "the gods just handed you a look that can't be duplicated" kind of thing. (Though not the look for the corporate CEO - I need a beauty/cosmetics or music video sort of project for it).
While that's a rare outcome, I do recall there's some famous celebrity-portrait guy out there who considers his fungus-trashed lens to be a secret weapon.

