They're a significant fire hazard which is why ATF regulations for stills require them to be located at least 100 feet from a residence.

I live in a city with 2 distilleries. You can smell when they're dealing with the mash because everyone in town can smell it. Also, we all get some black mold (not the really bad one) all over our siding which I think is some byproduct of the fermentation step.

My father worked in the oil business. As a chemical engineer, he was brewing his own moonshine (Poitín [pronounced 'poh-cheen'] in Ireland, Sidiki [means 'friend'] in Arabic language countries) since he was in university. In Saudi Arabia, there were frequent home fires in the western-expatriate communities. Newspapers reported them as "unattended cooking pot" fires. It happened several times per year in the Ras Tanura community they last lived in.

I would say: not explosive. I've seen a decent number of setups, and I can think of three areas where you could be concerned with safety (not necessarily where you should be):

1. Most use propane burners (the exact thing you'd use for homebrewing which is already legal and safe, and also similar to what some large turkey fryers use) which can be risky, but some are electric (120v or 240v).

2. Stills are an open system insofar as there is a way for pressure to escape - if you're goofing things up, you might vaporize and not recondense your ethanol (eg, because you have the heat way too high and/or aren't doing a good job of cooling down the vapors), and it's possible for that vapor to start on fire. I've seen it happen, and it's certainly a spectacle but wasn't particularly dangerous.

3. The distillate itself (ie, ethanol) is usually pretty potent, especially the foreshots and heads. Let's say 70%+. Especially as it's coming out, it's still prone to evaporation, and you could have a combustible/explosive risk here, but I've never seen this to be an actual problem.

Not explosive, but still a potential fire hazard, especially if a still gets way too hot (coolant system fails) and alcohol vapors escape. The risk becomes extremely minimal when using an electric still.

they're really not. they're generally not a pressure vessel, and even when I've had leaks of ethanol, the fire went out immediately after being removed from heat.

today's home stills are usually plug-in resistance heated chambers with a still head, and are very high quality. my flame-out was from a pot still that was sealed with flour and water, not a modern still.

less than a meth lab