This topic has been covered in the past on the previous community forum and there are many bullet points to cover, but I wanted to open up the topic for thoughts, problems, questions, etc.

I have come across many scenarios that encountered an issue with out-of-the-box solutions. I was left with diving into how to customize to meet our requirements.

The biggest thing I would like to point out, which I believe is outlined in the FAQs, is that many are inclined to default looking at the JavaScript API endpoints. This option eventually leads having something missing that may require you to hook back into the server side API.

One thing to note, I found the best option is to leverage WebAPI built into the CMS and make your own endpoints so that you can make precise requests and responses for your snappy UX, yet still get practically full access to the functionality needed to customize your cart experience without changing source or workaround issues. This gives you all the control by referencing HCC libraries and consuming the user context so that you can control what happens client side and server side if you choose to use an extension point.

Feel free to propose some scenarios and/or problems and I'll be happy to provide some examples.

Happy Coding!
Adrian