On the bike I use a Tickr HR strap and wear a Garmin Vivoactive 3 ‘watch’.
Today on the trainer doing thesthold work the strap said 176 while the Garmin optical said 88, and never rose above 116. The optical HR figures for me on the bike are just nonsense, and so I make no use of them whatsoever.
When running, the Garmin optical has been better, but only up to a point, frequently under-reading, sometimes dramatically. Sat around, the figures from the Garmin optical seem correct.
But for my partner, the same Garmin optical wrist-based seems to work fine for her when running, every time.