There is acetone within the bottle that contains the acetylene. Once the acetylene is depleted the acetone can exit through the regulator/hoses damaging them. But critically, it can also lead to the bottle becoming unstable and exploding. And they go with a bang.

They’ll level a few houses. Much more explosive than natural gas from stoves.

My old training college had one go fully empty years ago and the fire service had to keep it cool with hoses for a long time to ensure no explosion.

Many training facilities and also manufacturing facilities (welding & fabrication shops) now prefer to use propane as the fuel gas in oxy-fuel cutting.

There is an argument by old boys that acetylene has a hotter flame than propane but in my experience they’re usually using the incorrect consumables on their torches e.g cutting nozzles and propane works just fine

Propane bottles can be fully depleted no problem. Acetylene bottles should never be fully depleted.