The major problem with used military Hummers or Humvees for civilians is that they are not sold with a title and are specified for off-road use only, which means they cannot be registered or driven on the streets. They may be illegal even in some off-road locations.
Surplus military Humvees, are only offered for sale on rare occasions. They lack proper safety requirements for road travel, although in some states they may be able to be brought up to standard with modifications. This means that any time they need to be taken over public roads, they must be shipped or towed by another vehicle, which can be cumbersome and expensive.
Military Humvees are also sold as-is. Although most of the trucks sold were used for light duty on bases rather than combat, they may still have engine or other problems that need to be fixed. Military Humvees do not meet federal emissions regulations for private vehicles. The interiors may have been stripped before the sale, depending on how much military equipment was in the Humvee.
In military use, Humvees have problems because the engines and frames are not really designed for heavy armor. Their slow speed and heavy weight also makes them an easy target for improvised explosive devices.