For middle class families whose income comes from wages as employees, it is not especially complicated. Be happy.
It is a lot harder for lower income families that may be receiving different forms of needs-tested benefits, small business owners, independent contractors and so on. Even figuring out how many people are in the family is not such an easy question when you may have nieces/nephews living with you or multiple families sharing housing.
One benefit of using 2 year old tax data is that schools could package financial aid a lot earlier than many of them are able to do right now. I believe that an earlier analysis of this proposal showed that the variance in Pell grants was very small using that data vs. last year’s data.