If you do it right the risk of fire should be no more than with any other electronic equipment. Your miners and power supplies will fail long before they get hot enough to go on fire. The biggest risk is probably the wiring in the walls, but if you have a problem there you are at risk no matter what you plug in.
Agree on this, if people are using a lot of power then they need to make sure their cable is able to withstand the amp that they pull. If not it would be really dangerous even if its normal household item like air conditional.