The short answer
Lenders assess how easily a property can be valued, insured, and resold. Higher-risk properties are more likely to be declined.
A mortgage decision isn't really about whether you can afford the property; it's about whether the lender is confident it could resell the asset if it had to. Anything that makes that harder to predict makes a decline more likely.
Why this matters for your purchase
PropM8's risk rating rolls up the building safety and environmental factors lenders care about, so you can see how a property is likely to be viewed before you apply for a mortgage on it.