The short answer
Legal issues often require clarification, indemnity insurance, or lender approval, all of which add time and uncertainty.
None of these things are necessarily deal-breakers on their own, but each one adds a round of back-and-forth between solicitors, lenders, and insurers. The later they surface, the more that back-and-forth eats into an already tight timeline.
Why this matters for your purchase
The title review section of a PropM8 report exists to surface issues like these before solicitors, lenders, and insurers get involved, while there's still time to plan around them.