By nature, moonlighters lack state HVAC licensing, do not carry Workers' Compensation insurance for employees, and almost always no liability insurance. Usually, they can't obtain any required local or state building permits because they are operating illegally.
If an unlicensed handyman or moonlighter makes a mistake, such as accidentally burning down your home when trying to solder a copper line with a torch, he is unlikely to have insurance to cover the loss. You may be stuck with the deductible and if your insurance replacement costs are not up to date, you may not be able to fully replace your home and possessions.
When uninsured moonlighters or handymen injure themselves while working on your property, you are legally responsible. You might be liable for their medical expenses, compensation if the individual cannot work, and for pain and suffering.
If the unlicensed and uninsured moonlighter fixes a symptom and not the problem, or performs slipshod work, you may have no recourse to get them to return and perform the repairs correctly. If you use a quality, HVAC state-licensed contractor and the work is not performed satisfactorily, you can contact the state HVAC licensing division and request an inspector to check the work to ensure it is done according to all codes.