I have always understood that everyone agrees that he is one of the greatest sci-fi writers ever. His writing and philosophy has depth and he understood the future would be interaction between the society and technology.
Maybe he is underrated in the sense that while he is considered a great, many of his writings are still relatively unknown.
His nonfiction is also remarkable (Golem XIV is technically sci-fi, but it's so philosophically heavy that it can be seen as philosophical work).
* https://en.wikipedia.org/wiki/Summa_Technologiae
* https://en.wikipedia.org/wiki/Golem_XIV
'The Philosophy of Chance' has not been translated to English.