Even if you train for HR, the Stryd will often make it easier. I use one, and when running up and down slopes, it is much easier to keep within my heart rate targets when I keep an eye on power.
I think Scott Johnston has written somewhere in this forum that the Stryd shows wrong power when you walk very slowly up steep slopes. I can add that it also shows wrong power when walking very fast. When the walking pace is faster than approx. 7:30 minutes/km (12 minutes/mile), it seems to pass a threshold where the power will suddenly increase. And when the walking pace gets even faster, it passes another threshold where the power will suddenly decrease. This can be a little annoying if one uses fast walking for aerobic training.