I have noticed significant differences in the HR data displayed on my Watch3 and what is shown via the Samsung Health app on my Galaxy S8+ (and this is after synchronising)
1. Peak HR is different - often by 15% or more
2. Amount of time in each of the HR zones is frequently completely different, for example following a "Walking" exercise, where I have triggered the start of the walk, manually. Example from yesterday, on the Watch, at the end of the walk (when I stopped the recording) it showed significant amount of time in "aerobic", some time in "Weight control" and some in "Low intensity". On the app, it just shows a very small amount of time in "weight control" and most time in "low intensity".