The junctions in TECs are of Fe strips soldered on the opposite ends of the semiconductor cubes and are held in place by the ceramic plates.
The Thermal expansion becomes a major factor when you try to maintain large temperature differences. The Junctions of cold side shrink and the junctions of hot side expand. The unit will suffer a mechanical failure way before T(hot) reaches the melting point of the semiconductor.
Since it’s a series of diodes by definition (mostly 127), a single failed junction renders the TEC unit useless.