Eight cases of TASER X26 ECD-induced loss of consciousness were studied. [...] This report offers evidence regarding the mechanism by which an ECD can produce [reactions] leading to cardiac arrest.
(I've tried my best to make it parseable without obscuring the general meaning. The original text is linked above and here.)
TASERs are generally treated by law enforcement as the standard for safe, reasonable option for dealing with civilians who seem like they might be dangerous, but who haven't presented any real cause to worry. And while I understand that police are in a stressful position, it's just not reasonable to expect everyone to have uniform reactions to a police officer. It therefore isn't reasonable to treat any potentially threatening action as worthy of potentially lethal force.
TASERs are potentially lethal force. That's beyond doubt. And personally, I don't think it's reasonable to use them as a go-to tool of law enforcement.