I can't speak for Ren's nightmares, as he refused to open up about them, but given the scar on his forehead... I suspect I know what one of them entailed.

[He grimaces. Even if Akechi didn't truly kill Ren, he would have, had the Phantom Thieves not outwitted him.]

On the whole, the nightmares were incredibly pervasive, only truly subsiding once I opened up about the matter.