Why do uPVC and timber windows fail in different ways?
Because they fail through different mechanisms: timber decays, and uPVC's failures are almost entirely in the components rather than the frame material.
How timber windows fail:
Rot, where moisture gets into end grain and joints and stays. The vulnerable points are the bottom rail and the joints at the corners, where water sits. Paint failure is the precursor — once the film cracks, water enters and cannot get out, and the coating traps it.
Movement with humidity, causing sticking in damp weather.
Putty failure on single-glazed units, letting water to the glass rebate.
The redeeming feature: timber is repairable. A rotten section can be cut out and spliced, joints can be re-made, and the window can be repainted indefinitely. Well-maintained timber windows can last a century or more, and historic examples routinely do. The problem is that maintenance is required, on a cycle, and skipping it for a decade is what destroys them.
How uPVC windows fail:
The frame itself is durable — it does not rot, and it needs no painting. Modern profiles are internally reinforced.
Sealed unit failure is the classic. The perimeter seal between the panes degrades, moisture enters, and the unit mists internally and cannot be cleaned. The unit is replaceable without changing the frame, which is a repair many owners do not realise is possible.
Hinges, handles, friction stays and locking gear wear out, and these are usually the actual reason a uPVC window stops working. Replacement parts are widely available.
Gaskets and seals perish, causing draughts.
Discolouration and UV degradation on older or cheaper profiles; modern formulations are much better.
Sagging and dropped sashes on large units.
The honest comparison: uPVC requires almost no maintenance and has a finite life, typically a few decades before the whole unit is uneconomic. Timber requires regular maintenance and, given it, effectively unlimited life. In a conservation area or listed building, the choice may not be yours.