Yes, a badly-operated still can allow poisons into the product, but a well-operated illegal still cwill give a product just as good as legal commercial operation. It’s only chemical engineering, isn’t it?
Opperating a still is easy as an easy thing.
Gin for example is just a single stage batch distilation.
Vodca is (usualy) a 7 stage batch distilation with carbon filters downstream.
Wiskey/whisky is (usualy) a 3 stage distilation
etc
etc
etc.
All fairly straightforeward, you could do it with a pressure cooker and some copper piping.
Unfortunately buying paint thinner and badgeing it as vodca is even easier. IPA, methanol, heavies, etc would only present in small quantities even in badly distilled vodca, the problem is it’s quite easy to buy these as industrial chemicals and mix them into somethign that tastes like cheep vodca.