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.