Got anything to back up that claim? John McWhorter argued pretty persuasively in "What Language Is" that the complexity of some morphological nightmares like Archi and Navajo is unparalleled by analytically complex languages.
Actually, I can't seem to find much on the topic. Geoffrey Sampson argues in
Language Complexity as an Evolving Variable (2009) that this viewpoint is wrong (he is a major critic of linguistic nativism in general, such as the ideas of Chomsky and Pinker). However, Miestamo et. al. discuss the difficulty of objectively measuring the complexity of a given language in
Language Complexity (2008). I haven't read the book by McWhorter but I'm not surprised he would take that view, since he specializes in creole languages, which seem like an exception to this assumption.
Fromkin et al. argue that all languages are in fact equally complex and have the same expressive power in
An Introduction to Language (2011). They appear to take the Chomskian view of linguistic nativism. I believe Sapir argued in favour of all languages having an equivalent amount of complexity as well.
It appears to be uncontroversial that languages can be more complex than others in certain aspects; there may indeed be no equivalent to the morphology of polysynthetic languages like Navajo in analytic languages, but that doesn't necessarily mean Navajo is more complex as a whole.