The short answer
Fire-safety concerns, cladding materials, building height, construction type, or missing certification can limit lender options.
Any one of these factors can narrow the pool of lenders willing to offer a mortgage, and several together can rule out mainstream lending altogether. It's rarely obvious from the flat itself, since the issue usually sits with the building, not the individual unit.
Why this matters for your purchase
PropM8's building safety section is built to flag the building-level factors that narrow lender appetite: construction type, height and fire safety, before you've committed to a flat.