I think his famed "sadness" was the disappointment in the reception of Snyder's vision. I know Affleck had a lot of emotional stock in it (as do I). I would be surprised if he felt his individual performance was being heavily criticized.
This is why we need a Snyder Cut. I bet you his Batman was just as great in the Snyder Cut of Justice League.
I wanna have 'faith' in Reeves, but at this point with Worlds of DC seemingly in shambles (Affleck gone, seeming indecision of Cavill, etc), we need better than drek casting to rise interest in future productions. This isn't the MCU with tons of good will. This is a universe with tons of divisive, weird, and downright poor decision-making.