Will Unitarian Christians and Jews go to hell?

Quran 2:82 says that the people of the book will go to heaven if they believe in Allah and the last day etc. But, Ṣaḥīḥ Muslim 153 says that if the Jews and Christians hear about the Prophet and they don't accept Islam they go to hell. I assume there are Unitarian Christians and Jews throughout the years that have heard about the Prophet, but did not accept Islam because they didn't hear enough about him or they were confused on certain things and they never got their questions answered. Is it fair that these people go to hell eternally?