Keepers are so hard to judge, I mean look at Liverpool's bad luck over the years. Karius and Mingolet looked good for periods, but the mistakes kept mounting.
Mendy looks like he has a howler in him almost every game I see of him (not that many admittedly). He's a strange one, cause he pulled off two top saves against us in the 0-0 draw, but almost passed the ball into his own net that day.
There's just something about him that doesn't fill you with confidence. That being said, even if he keeps Kepa out of the team then that's huge. Hard to pick between Kepa and Pepe in terms of the bigger flop.