The biggest issues I have with Haki are that it takes up too much real estate and thereby undermines Devil Fruits and potential for interesting battles around creating solutions, the advanced forms are very poorly thought out and poorly implemented, and the general lack of consistency in the use or explanations of Haki abilities add more issues than they solve.
I feel oda is deliberately keeping haki confusing.
I totally can understand why he needed another power system like haki
1) to emphasize will power and inheritance of will
2) dfs though could have given us much more interesting fights but involves lot of creative challenges in thinking about their usage. Not to mention, drawing them would be pain in ass as well.
3) haki gives way to be powerful to those who can't or don't want to eat dfs - like shanks.
This is important as many in Op world do consider eating DF is bad as it comes with a curse of being a hammer in the world of seas.
So, I do feel oda really hit the nail with haki concept...easy to draw and manifest as ideas
However, the execution is pathetic.
People really don't look for deeper explanations as long as power level remains consistent and well - delineated.
But with haki - its just confusing.
Sometimes character uses CoO, sometimes doesn't.
Too much brain storming regarding what's advanced and what's not.
DBZ just through hair style change gave us clear delineation.
Even a simple change in haki color to show variations would have done the trick yet somehow oda keeps it entirely confusing.... I definitely believe he's aware of this but if he still keeps it like this then it's a deliberate choice