All of the methanogens are obligate lithotrophs - they can make energy only by methanogenesis. The enzymes in the methanogenic pathway are very oxygen sensitive, so methanogens are extreme anaerobes. Methanogens are common organisms, found in all types of anaerobic environments, and are certainly the most prevalent Archaea in the 'moderate' world. Methanogens are crucial components of the micobial populations of the 'rhizosphere', the plant root environment. African termite mounds are scruoulously aerated by the insects not for oxygen, but to keep methane concentrations low.
Cows also produce large amounts of methane.
Several houses near older unvented landfills have exploded because of the buildup of methane that seeped through the ground into their basements. Oil deposits - natural gas is methane, produced not geochemically but by methanogens living in the subterranean oil deposit. Methanogens form a variety of symbioses with plants, animals and protists, but despite these close associations there are no known pathogenic methanogens. None of the other Archaea are pathogens either, but considering the conditions under which they grow, this is not surprizing. Methanogens associate with these organisms, utilizing the hydrogen they generate for methanogenesis, and keep the hydrogen concentrartion low enough for the heterotrophs to make a living.