It's really nothing more than a fancy guesstimate. Cool to look at, too unreliable for anything real.
If you really have any health concerns, use a cuff whenever you need to know. If you don't, you won't care about this feature anyway. A number is worthless if you can't count on it.
The ECG on the other hand, that's kinda cool. It's only one channel and it only looks for one kind of defect but at least it does that fairly reliably so, as long as you sit really still while measuring it to get a clean reading. It holds no candle to a real multichannel ecg but at least it actually measures something.
So you still need a wear a sphygmomanometer: https://en.wikipedia.org/wiki/Sphygmomanometer