Right-click would be equivalent to long-press on iOS. But don’t long-press too hard or you’ll get 3D Touch. Or maybe you do want 3D Touch, so you press hard. Surprise! Your iPad doesn’t support 3D Touch; you were thinking of your phone. Does this particular app even support 3D Touch? Only one way to find out!

Inconsistent and not discoverable, which isn’t anything like a right mouse button at all.

You can right click almost anywhere and get some feedback. Almost every element has some context menu. That's not the case for 3D touch. You just have to know in advance where it works, experimenting won't work.

Also I agree that mouse has tactile feedback. Even if I wanted to experiment with 3D Touch, I can never know whether there's nothing 3D-touchy under my finger or I just did not apply enough pressure.

IMO they should have some standard UI idiom for 3D-touch enabled regions and they should have negative feedback (like some vibration) when user pressed 3D-touch on unsupported region. But in its current iteration 3D touch is almost user-hostile. I can't imagine why Apple released it without better polish.

FWIW I know about it, am not old or unhealthy enough to have fine motor control problems, and can't use it effectively. I have to disable it to get press-and-hold to do what I'm used to it doing, reliably, without requiring several attempts.

Ditto the similar force-press thing on the newer Macbook touchpads—my file drag-and-drop operations drop to a success rate of about 50% with that enabled. Took me a while to figure out what was wrong with my new laptop first time I used one, because I'd forgotten about that "feature".

There's a physical button on the mouse to aid discovery, which gives you tactile and audible feedback that it has been actuated, and whose method of action can be discovered by observation of others (you don't have multiple identical-seeming actions which produce distinct results).