Where lead usually fails
Corners, long rigid sections and areas pulled tight around fixings are common weak points. A small split may be hard to see until the lead is cleaned and inspected closely.
The chase above can leak
Lead may be perfectly sound while the mortar joint holding its top edge has opened. The reverse is also true: new pointing cannot compensate for a split piece of lead underneath.
Patches have limits
A local repair can be sensible on otherwise sound lead. Long runs showing fatigue in several places are different. Covering widespread splits with bitumen makes later inspection and proper leadwork harder.
Keep pieces manageable
Traditional lead detailing divides larger areas so sheets can expand and contract. A replacement should respect that movement rather than create one oversized rigid sheet.