Really depends on what you mean by “toughest”.

For me, it would be The Fountainhead by Ayn Rand. I was doing a buddy read with a coworker and it was incredibly boring. I could only read it in 70 page chunks before I’d doze off.

Jen Campbell. She is an author and has worked in the publishing industry. She talks a lot about disfigurement and disability (she is disfigured [her words] and disabled). She primarily reads adult books in a wide variety of genres.

It really varies depending on the story being told.

But I often feel that authors resort to extra POVs needlessly. Especially if there a few main perspectives and then there is one random person for 2 pages that you never see again. IMO it’s lazy writing.