Not very lethal, especially compared to swords and spears.
Lacking reliable weights for old intended-for-combat Chinese meteor hammers, let me look at the Japanese versions: the fundogusari (weight-chain), kusarifundo (chain-weight), ryofundogusari (double-weight-chain), and manrikigusari (ten-thousand power chain). These usually have weights of mass 60g to 120g (and the same is usual for weights on kusarigama (chain-sickle)). These weighted chain weapons were designed as less-than-lethal weapons (to use the modern term) and were used as police weapons.
Amdur (2014) discusses sparring with kusarigama, and notes their relatively low lethality. While less-than-lethal, they can kill (avoid hits to the head and neck!), and you certainly don't want to be hit by the weight even on less vulnerable parts of the body. However, compared to the damage that can be inflicted with less force by a sword or spear, "less-than-lethal" is an appropriate description.
Supposedly, the weights on the Chinese weapons were often heavier than those we see on the Japanese versions. I haven't seen any weights for old examples intended for combat. Weights for modern martial arts training/demo weapons are easy to find, and even weights for fake antiques, but neither is reliably informative about old combat weights. Old ones don't have the giant sized weights (typically hollow) we see on some modern training/demo versions, and in movies. An example from the late Ming Wubei Zhi:
https://commons.wikimedia.org/wiki/File:Meteor_hammer_2.JPG
which has relatively petite weights. Heavier weights will make hits more damaging on average (but practical weights will still be much less lethal than swords or spears, even if heavier than the 60-120g typical for Japanese weights), but will make the weapon more predictable and thus less likely to hit.
Mol (2003) discusses the Japanese versions in detail.
References:
Ellis Amdur, Old School: Essays on Japanese Martial Traditions, Freelance Academy Press, 2014.
Serge Mol, Classical Weaponry of Japan, Kodansha, 2003.
