In the wiki page about unsuccessful investigations of Einstein, there is only one sentence describe his unsuccessful of unifying general relativity with electromagnetism:
Einstein spent many years pursuing a unified field theory, and published many papers on the subject, without success.
It has more detail in the Einstein's wiki, but only focus about his being isolation from mainstream physics. However I can't find any info about what we have learnt from his unsuccessful. By learning I mean the physics knowledge, not life lessons like "Never give up" or YOLO. So far the only thing I can only tell one thing about that is the concept of unifying physics that Einstein had inspired other physicists.
