+ obviously adds a row, I hope we can agree on that! If you just tap it, you get another row right after the current row, at the same level of indentation. That's the most common case, so it makes a good default, I think. The other choices (which appear if you tap and hold, but can be immediately taken by swiping in the right direction) let you point where you want the new row to go, relative to the current one. How do you propose to offer those 5 choices in a fashion that is both more intuitive and at least as fast and easy to use? I don't think lining them up along the toolbar is it, because that decouples the gesture from the meaning and adds confusion. There are two choices for putting in a row at a level to the left of the current one; which one comes first, the button that makes a new parent or the one that makes a new child? Now instead of a set of related gestures anchored in the same spot, you've just got a line of buttons, and what have you gained?
While I won't disagree that figuring out all the uses of that widget without any recourse to a demonstration or documentation might be initially challenging for some, it is the case that you get both built-in documentation and online video demonstrations to help you over the discoverability hump. That control is something you are going to use frequently if you do any actual outlining — or, it's something you don't need at all! If you use it, you'll remember it once you've learned it, and if you don't need to use it, you don't need to remember it. For what it is worth, you can work pretty quickly just using the + button and tab in|out buttons in many cases, if you don't like the + "constellation".
It seems suboptimal to base the design of an important part of the user experience on the convenience of a set of users who don't use the tool very often vs. the convenience of the users who want to work it hard, don't you think? After all, presumably most people are shelling out the usual premium Omni price at least in part because they want a powerful tool and plan to use it accordingly. Users who want just a bare-bones list-making tool where they don't need to invest any effort in learning have a number of inexpensive apps to choose from, but there's only one OmniOutliner for those of us who want the whole 9 yards.