[Edit] Both of them make Dan Brown look really, really unimpressive. If "The Da Vinci Code" is a vaguely thought-through concept aimed at general readers, "The Illuminatus! Trilogy" is an explosion in a conspiracy theory factory.
To be clear (for the benefit of others), not specifically of Dan Brown. DB was only aged 11 when Illuminatus! was first published in 1975.
It's mostly because Browns work is just a cheap simulacrum of the original conspiracy theorists who came up with the story: https://en.wikipedia.org/wiki/The_Holy_Blood_and_the_Holy_Gr...
In the book there is the description of a fire in Paris on a specific date. The reader wrote to Eco, that he must have gotten the date wrong, as he was at that place that night and that there was no fire.
Eco uses this as an example, that some readers do not understand the signifiers of fictionality and that authors sometimes have to deal with readers taking their fictional work literally.
https://slatestarcodex.com/2018/10/30/sort-by-controversial/