Heart Rate Way Wrong

This is my second post about this device and I'm starting to scratch my head about why I spent the money.

I worked out this a few mornings ago-- hard rowing workout. My HR was recording a BPM of around 90 -- way, way lower that it actually was. My actual HR was around 150-160.

Last night I took a really brisk walk, including up a long hill. 5X+ showed a heart rate of between 59 and 65. Actual HR was around 110.

So what's gives?

Any ideas about why this would be so far off? I selected the "row indoor" activity, obviously. I tried both wrists.

Between this giant inaccuracy and the useless (and very misleading) sleep tracking, I'm starting to think this device is no good.

? Things can't be this off.