What about prop planes? It’s my understanding that the problem lies in the silica in the ash plume fusing with super heated jet engine blades.
The silica will score cylinders and rings. The damage will not occur as fast, but it will occur.
Also, windscreens and other surfaces can be badly damaged.
The time saved over rail or boat travel would be eaten up (literally) by damage to aircraft.
And the safety aspect cannot be ignored.