These words can correctly be used synonymously, but in some situations their meaning differs. Modern should be applied as an adjective to anything that exists in the present age, exists now in contrast to existence in an earlier period: "Becky cannot adjust to modern city life." "Bob prides himself on his modern viewpoints." "That building is an excellent example of modern architecture."Modernistic can be used as modern is, but it is more often applied to items or ideas that have been, or may be, short-lived, exaggerated, or experimental: "I like up-to-date styles, but this furniture seems too modernistic to me." "His life-style is so modernistic as to seem unorthodox and even incredible."
Dictionary of problem words and expressions. Harry Shaw. 1975.