The main reason that gross national income figures can be misleading is because they

It doesn't consider differences in the cost of living.

Explanation:

The Gross national income is a cumulative measure of the total income of the residents of a country in certain period usually a year.

It is misleading since it does not indicate the individual cost of living of residents. Remember, the individuals in the economy could be paying high taxes that eat up a greater part of their income, though it might seem as if the GNI value is good if you based your analysis of the economy on it.