Flat mount is supposed to eliminate the need for adaptors... It doesn't...
It also over complicates the frame prep and installation.
Put it this way, post mount is fine as it is, so is IS but manufacturers just have to create some new stuff for the sake of ' improved and better ' so you'll buy more!
Flat mount advantage is that the rear triangle doesn’t need to be re-designed and can be adjusted under the chainstay instead of awkwardly inside both chainstay and seat stay.
Flat mount perfect for road. Post mount for MTB/hybrid.
Flat mount is supposed to eliminate the need for adaptors... It doesn't...
It also over complicates the frame prep and installation.
Put it this way, post mount is fine as it is, so is IS but manufacturers just have to create some new stuff for the sake of ' improved and better ' so you'll buy more!