Yes, it is not as good a HR monitor as your typical chest strap, and frankly there are better optical monitors out there, but they tend to be in dedicated sports watches.
The S does a reasonable job. However, the exercise apps do seem to be some of the biggest battery drains. If you really want a good exercise watch I'd be more likely to go with something in that space. Personally on rides I have the S on my wrist for communication, but use the Garmin mounted on the bike for tracking.
My wife sometimes caries the S with her on runs, but the Garmin is on the wrist, and the s in a pouch just in case.