Off the top of my head, the top failure mode is house fire. Either the sheathing eventually catches due to pyrolysis, or a chimney fire speeds things along. If he's lucky, he'll get some melted siding as a warning sign first, so he can jam a big rock in there.
Other stuff:
The ''insulation" is pink fiberglass, which decays into gravel under heat and has an outright melting point of much less than a chimney fire.
Vinyl siding and plywood sheathing within 2-3" of the flue. No insulation on this part, not that pink fiberglass would do anything for long.
Soaking wet insulation all summer causes rust-through on the black flue pipe below; water runs downhill so the worst holes develop right at the bottom.... by the combustible house.
Galvanized cover emits toxic fumes when heated.
~12' of rise, ~10' of horizontal, 2 90s, and another slight elbow. Draft is gonna stink on a good day.
From the audio, it was previously uninsulated and at a much lower angle.
Also from the audio, he just built the house so recently that he scavenged the insulation off the floor. In the middle of new construction, when it would have been easy to do anything, THIS is the venting solution that he chose. This understanding leads us to another likely failure mode, which is that this guy decides that it would be better to just vent the stove upstairs through the HVAC ducts so all the heat stays in the house.