Why do people call Index a bad anime adaptation?

I first watched "Index" in 2013 and was excited when Season 3 was released in 2018. I've only read nt1 and nt2, along with the beginning of nt 3, and I've read the manga that covers the content of Season 3. However, I haven't read any of the ot light novels. How bad is the adaptation, and how much has been cut from the anime?