I'm not sure it's worth a lawsuit but ... the students may have a valid complaint there.
My school is in the process of switching from ATI to HESI because the ATI deviates quite a bit from the way they teach nursing at my school. HESI is actually based a lot more on the textbooks we use than the ATI.
To say ... "try studying more" is oversimplying the problem. The fact is: when you do study you can go to three different textbooks and get three very different answers to the exact same question. Nursing academia is rarely consistent in giving students the "right" answers to test questions, even when you do study.
However, having said all of that ... the same problem happens with the NCLEX. And ... whether we like it or not, no one can challenge the varying testing styles on that exam.
So, I guess we just have to deal with it.