It is not discoverable. It is a power-user feature that makes the UI less obvious for ordinary people.
A lot depends on how developers use/abuse this feature. If only power-user features are hidden in the 3D-touch menu then it will be OK. But if ordinary features are only exposed via this menu then apps will become harder to use.
I don't think so. If the ordinary user doesn't know about it, they won't care. They'll still continue to press as normal. When they do discover it, they'll try it again to see if they like it, if they don't they'll forget about that too and continue on with regular presses.
Just like the watch. I may be stupid but I really don't like that the watch makes me feel like I definitely am when I can never find or repeat anything on it. It's a really lousy feat of experience design
A lot depends on how developers use/abuse this feature. If only power-user features are hidden in the 3D-touch menu then it will be OK. But if ordinary features are only exposed via this menu then apps will become harder to use.