First off, this is not a glass nozzle, it is a whole hotend design. A super simplistic one.
Glass is, like ceramics, not a good thermal conductor but has a quite good thermal resistance - it only melts at about 1600 °C, which means you will never have to fight melting or warping of the filament path itself at all - the heater copper wire will melt at about 1084 °C, so way before the glass, and most plastics that are printable start to decompose at less than 400°C.
Construction-wise, this design has some benefits:
- Due to the design and material properties, this hotend doesn't need cooling fins and a "coldend" is not needed at all.
- The whole hotend being one solid piece makes it pretty much a "plug and play" item and prevents leaks.
- Glass is extremely abrasive resistant. This means a glass nozzle could be used for stuff like carbon fiber filament very long.
- Glass can be molten, repaired and modified with fairly simple equipment, e.g. a burner and some skill.
- Glass could be easily cleaned up to medical and food-grade machine ratings. The simplicity of the hotend assembly could make it autoclavable as a whole piece.
It has some downsides though:
- Glass is brittle and does not take lateral forces and sharp impacts kindly. In other words: Handle with extreme care.
- Due to the glass being an insulator, the inside of the hotend will have a lower temperature than the outside.
- A fairly thin-walled meltzone could mitigate this problem to some degree at the downside of making it even more prone to breaking
- The insulating behavior means, that the meltzone has to directly feed into the nozzle with as little unheated area as possible to prevent the molten plastic from solidifying inside the nozzle again.
- The skill needed to create a properly sized nozzle from glass is tremendous.