Even if the market is super hot right now and it seems like people will buy anything, anyone who looks at buying this will definitely be thinking about how hard it will be to sell in 10 years when the market isn't so hot. Most likely 90% of them will either back out or demand discounts that basically assume the whole thing is a mess and will need to be ripped out by professional contractors.
They list all kinds of small details: a single double glazed window containing condensation, or a gutter leaking onto an exterior wall. Experienced buyers would know which of the details are actually important, but it's still a sobering document. They encourage buyers and mortgage providers to act cautiously and are typically used as a tool for negotiating the price down.
If you are a cash buyer and just don't care, then fair enough. But whether or not it's a sound piece of engineering, a DIY tunnel (without planning permission?) dug underneath the property would be a massive red flag for your solicitor and your surveyor who would likely provide professional advice against buying the property. And mainstream mortgage advisors and insurers and may even refuse to do business with you due to perceived risk.
That said, I have trouble believing in the current insanity of urban property markets that anything could make a regular house hard to sell. It won't make it more valuable (nobody's shopping saying "I really need that underground bunker") but I seriously doubt he'd have any trouble beyond that.
Genuinely curious to why that would be? Subsidence?