I feel that what makes hard sci-fi interesting is not necessarily the plot but the exploration of the themes and the imagery that they prompt you to develop. They tend to be very densely packed with thought-provoking ideas. I remember reading Greg Egan and having to stop all the time to think about a paragraph.
It doesn’t help that they use a lot of scientific and technological terms. You must have a very good grasp of the language to enjoy this genre. It took me about a decade after starting to read English scifi in original to be able to read it properly. Thankfully the mainstream classics are a lot more accessible.