While they can provide heat, miners have 4 drawbacks:
1) usually no on/off switch
2) not child or pet friendly usually
3) not as cheap as natural gas in some areas for heating
4) Most importantly - hot PCBs and components give off VOCs that pollute the air - these are known carcinogens. 1 miner running 24/7 probably is no worse than new carpet. Having a farm running in a house with no fresh air could cause permanent detrimental health effects.
4) can be solved by an air purifer if you are concerned about VOCs.
Home air purifiers will not filter out the dangerous VOCs. They can ionize and capture free ionized elemental particles - they will not filter out VOCs. You would need a scrubber like those found in respirators or activated filters.
The only solution to this is constantly bringing in clean outside air. The self contained oil based space heaters don't emit much outside of their metal housing.
"Eliminate virtually 100% of your homes Volatile Organic Compounds (VOCs)." I don't know how true it is but thats what they claim.