He has written some very questionable things about pedophilia (from which he has since distanced himself): https://stallman.org/archives/2006-may-aug.html#05%20June%20...

To be clear: this does not diminish his contributions in the field of software! His ideas about Free Software have been visionary and are as important as ever. One can be brilliant in one field and a fool in another. This is actually very common among technical people ("engineer's disease"). We cannot expect someone to be right 100% of the time.