It is very clear about the fact it cannot detect a heart attack though.
Then there’s these slightly more sophisticated things: https://store.kardia.com/products/kardiamobile6l
The EKG, as far as I know, doesn't really tell you much other than whether it thinks you have a normal sinus rhythm, or something it doesn't recognize. Too fast, too slow, or whatever. It's also on demand, not continuous. You have to touch a finger from your opposite hand to the crown, while the EKG app is running.
Also, that Kardia advertising is borderline scam. It's a 1 lead EKG just like the Apple Watch. Six lead my ass, they make that claim because you can contort your body in enough ways to take readings from each standard EKG location. You can sorta do the same thing with an apple watch, though it's not physically as large so it wouldn't be as easy. What rubs me the wrong way about calling the Kardia a 6-lead EKG is that it cannot do 6-leads simultaneously, and I think that is a critical detail.
I had a Kardia myself before the first Apple Watch with EKG came out.
Perhaps the watch can detect deadly heart attacks (stopped heartbeat)? Although I’m not sure how much use that feature would be.
It’s not like you should expect it to be a full EKG :P