(Re-posted - I just discovered that Blogger posted it according to the date of when I started writing the post, not when I finished.)
There are three types of rationalists.
There are medieval rationalists.
There are 21st century rationalists.
And there are medieval rationalists living in the 21st century.
There are plenty of ideas and arguments that seemed perfectly rational in the twelfth century, but which have since been shown fallacious.
So, for example: Rambam believed that only a fool would deny spontaneous generation. That was a reflection of the scientific beliefs of his era, which a rationalist today should not accept. Many rationalist Rishonim believed that God's existence can be logically proven. But as far as I understand, in the world of philosophy, that is no longer true; at best, it can be argued to be rational to accept God's existence. Rambam believed that being a good Jew and receiving a portion in the next world is contingent on intellectual perfection, and therefore simple-minded people, children, and those making fundamental hashkafic errors simply cannot receive a portion in the next world. But this was a result of his particular hybridization of Greek philosophy with Judaism.
I have noticed a distinct group of people who consider themselves loyal followers of the Rambam, but they are medieval rationalists living in the 21st century. And as Rambam says, "A man should never cast his reason behind him, for the eyes are set in the front, not behind." Rambam did not believe Chazal to be infallible, and he would certainly not have rated himself that way, either. A rationalist today should follow Rambam's underlying guiding principles, not necessarily his specific application of them.