A driver will overheat when driven continuously at full power, whether open to free air or installed in a sealed chamber. This is true of any driver. The overheating condition isn't so much because the rear chamber is sealed in a particular basshorn, although trapping hot air in a small sealed chamber slightly increases the problem. The main thing is the heat from the voice coil radiates into the pole piece and is trapped by the ceramic magnet. This heat then re-radiates back into the voice coil, causing the glue to melt and the voice coil to separate from the former.
This is probably the most common failure mode for any loudspeaker, causing the voice coil to rub. Over-excursion is another failure mode, and cone tearing is yet another. But most failures occur because the voice coil becomes unglued and begins to rub, making the speaker buzz and vibrate, and eventually as the coil wears through, it will open and fail completely.
I think there has been some confusion, perhaps misinterpretation of the fact that cooling devices added to high-performance basshorns improve their thermal performance. This isn't because the sealed chamber traps heat. It does, but that's not the killer. The killer is the magnet structure trapping the heat. Even with air cooling, and with the driver fully exposed to refrigerated air, the pole piece surrounding the voice coil will get hot enough to cook on when driven to just a few hundred watts.
If you think about it, a speaker voice coil is applied several hundred watts, so it gets hot like a large soldering iron. Even if the speaker system is
very efficient, you still have hundreds of watts dissipated as heat. Take a theoretical 400 watt speaker at a very optimistic 50% efficiency level - You still have 200 watts of heat. This heat source is surrounded by steel and then covered by a large chunk of ceramic. This is a pretty good heat container, one that is almost made to hold heat. So one of the best things you can do is to get a good conductor of heat down inside the motor, in contact with the pole piece. Wick the heat out of the core and radiate it away.
The best thing you can do to improve thermal performance on a LABhorn is to mount a cooling plug on its access panels and insert them into the drivers. That's
much better than leaving them open and exposed to free air, because the plug is better at removing heat than air cooling alone. This also allows the horn to operate as it should, with its rear chambers intact.