Hi,
oTree is an home made library that I created several years ago.
That library is fast, flexible and optimized.
If you have ideas to make the display of the tree "nice" using only CSS, we are aware and open to suggestions.
But I'm afraid that oTree is the best choice and the only choice for our components. No other lib is providing such kind of asynchronous features and such level of flexibility.
Customization wasn't my primary goal but it is possible to customize the display. HikaShop do not use the same icons in the backend for example.
Sorry i know i am harsh on these things, do not take it personal 
I won't take it personally. But please provide constructive arguments.
I don't think that talking about "pain in the ass" won't make things go in the right direction.
Regards,