problem
A wide mix still needs to survive the moment left and right are summed together.
The chorus feels wide in headphones, but the guitar thins out on a phone and the bass loses weight in a club. That often means key information is living in phase relationships that do not survive mono.
Mono compatibility matters because many real playback systems are partly summed, poorly placed, or heard from outside the stereo sweet spot.
Stereo wideners, unaligned doubles, chorus effects, and wide low end can create cancellation when left and right combine. The result is not always obvious until the mix leaves the studio.
If the side channel carries too much of the hook or low end, the mono version can lose the exact parts that make the record work.
Fold the mix to mono while arranging, balancing, and mastering. Watch correlation and M/S balance so you know whether width is adding excitement or hiding a translation problem.
Meter Core helps keep mono checks fast enough to use during the mix instead of only after export.