Coal synthesis gas /town gas contains significant amounts of carbon monoxide, hydrogen, water vapor along with minor amounts of a slew of other compounds. Methane content is typically only a few percent while the hydrogen content could be up you 55%.

The carbon monoxide made it highly poisonous. Looking back in hindsight, you'd be pretty foolish to cook with hydrogen cyanide gas which is also flammable, but has a similar toxicity level as carbon monoxide. There were many documented murders. All you needed to do was just turn the gas on in the victim's room while they were asleep, then go over to the pub and wait.

CO along with cyanide, ammonia, and certain nerve agents, is among the handful of toxic substances with the potential to kill in minutes, before rescuers are likely to arrive.

The hydrogen makes it uniquely explosive if it leaks into a confined, poorly ventilated space. Hydrogen has a much higher coefficient of thermal expansion than air, and has a wide range of mixture ratios with air in which it can burn. It can burn in as much as 90 H2 /10 air where most other fuel gases won't. Gas lamps were know to explode.

Hydrogen also has a unique tendency slowly to leak past o-rings, threaded pipe joints, gaskets, and packings that would contain other gases. It will also cause embrittlement of many materials like steels, cast iron, and many plastics. Stress induced cracks in threaded pipe fittings can be an issue, for example.