Amazon is really the ONLY place worth publishing.
Sure, there are all those other places...but they don't push the volume that Amazon does.

Depending on what kind of product you are publishing, Amazon can be easy or complicated.
Sure, you could just upload a pdf and it will build you an ebook...but that is a crappy ebook.
I have found that books built from docx or pdf files were wonky when they were converted to an eBook.
The index doesn't work, the fonts shift, images don't behave, the layout has glitches.

There are several good programs for creating eBooks that look professional.
Take the time and create a quality product.