There are a lot of people who shoot the 5mg and 10mg Methadone tablets, and those that shoot the 40mg wafers and various orange/red Methadose brand liquids.
Britain, Holland, and other European countries and possibly elsewhere in the world you can be prescribed Methadone ampules for either IM or SC injection (everyone IV's it, the doc's prescribe it for SC or IM on paper). Some patients are maintained on injectable Methadone for years.
So, Methadone is shootable, and plenty of people worldwide do it. As far as safety and comfort go, pure Methadone ampoules still produce a burn at the injection site most of the time, as well as irritation and possibly redness and swelling. Methadone pills are some of the most unsafe pills that are regularly shot. Lets say OxyContin and Dilaudid are the easiest/"safest" of the RX pills to inject, Methadone is on the bottom (shooting pills is never safe). There are those that rank IV Methadone pills (5mg/10mg) as their drug of choice. Read up on the Diconal phenom in the UK back in the 70s. Dipap is almost identical to Methadone, chronic IV use leads to gangrene, amputation, collapsed veins, severe abscesses, etc as a regular occurance. Shooting Diconal into the femerol vein on the top of the penis became extremely common among male Diconal addicts, as they ran out of veins everywhere else.
The only thing less safe for addicts to shoot is Suboxone pills (which is also widespread, look at India and France).