Apple has a long history of not releasing hardware if it doesn't work well. I have a hard time imagining them putting so much time and money into developing the apple watch and then releasing it with an inaccurate heart rate sensor. I'm willing to bet that it will be the most accurate sensor of all the wrist bands/smart watches currently on the market.
Whether or not it can compete with other types of heart rate monitors remains to be seen.
It's an interesting concept you bring up.
And it really depends what you mean about "working well" and your expectations and criteria for judging "working well"
One could select many pieces of Apple hardware, where Apple had deliberately chosen to make it work less well than it could do if they had chosen something different.
The whole Apple concept of "form over function" is perhaps the best way to demonstrate this, in that Apple will make something technically worse than it otherwise might be, simply for cosmetic reasons.
A very simple, very basic example?
Apple use Laptop grade low power graphics cards in their very expensive iMac range. Not desktop grade graphics chips which are faster.
Why do they do this?
Why do they fit worse components than they could simply select instead?
They do it, so they can make the case of the iMac thinner
Apple have decided that "It's good enough"
It could easily be better, and work better, but Apple have chosen to make the product technically worse.
This is why some people, including myself with Apple and these points, as I don't like this compromise.