It's a bit disappointing to hear that, as I have to disagree quite heavily.
Reign in Blood is probably the most brilliant thrash record ever. It's an 11/10 if there ever was one. I'm genuinely surprised nobody talks about the most brutal song ever written, "Necrophobic", nor the insanely catchy combo of "Altar of Sacrifice"/"Jesus Saves". The only very slight criticism I can give this record is that I have a bit of trouble recalling moments of "Reborn" and "Epidemic", but as soon as I hear those songs, it all comes back.