We try not to write scary content, most chimney problems are money problems, not safety problems. The liner is the exception, and it deserves an honest explanation of why, mechanism by mechanism.
Risk one: the slow fire path (pyrolysis)
Behind your liner sits masonry, and behind that, wood framing. A crack lets hot gases repeatedly heat a spot of that wood, and here’s the treacherous part: wood’s ignition temperature drops with repeated heating. Fresh lumber ignites around 500°F; wood baked over dozens of fires chemically degrades (pyrolysis) until it can ignite below 250°F, temperatures an ordinary evening fire delivers. This is why liner-gap house fires happen years after the crack formed, in walls that “were always fine”. The NFPA attributes a large share of home heating fires to exactly this mechanism, failed or absent liners in solid-fuel systems.
Risk two: the invisible migration (CO)
Combustion gases carry carbon monoxide, odorless, colorless, cumulative. In a sound flue, all of it exits above your roof. Through a crack, some of it enters wall cavities and finds its way to living space, at low levels that never trip a detector but visit every fire, or at acute levels when draft conditions push the wrong way. Bedrooms above or beside chimney chases are the classic exposure geometry. A CO detector near the fireplace and in adjacent sleeping areas is the minimum-wage bodyguard here; a sealed liner is the actual fix.