a) you have different sensor types
b) you have different tracking times
For a) you have ECG, EKG, and Optical - Polar H10 is great for ECG, but it’s a chest strap hence uncomfortable. Optical is comfortable, but it’s not so accurate and then you have the EKG like Kardia
For b), I think it’s the important bit. It’s great to be accurate in the measurement, but you cannot predict whenever you will have a non regular sample. Hence, the ones that measure statically ‘whenever you activate’, they are accurate but you cannot rely on them. Any optical and some ECGs have 24/7 measurement, which gives you a good look at the rate of change
I’ve used the Kardia and found it useful as a training device to check whether my subjective experience of arrhythmia was reflected in the data. But I don’t use it often. Here is an image of the data it records, with an arrow where the there was an arrhythmia. (There is another, you can see, five beats later.)