I've been working recently with the STM32F769 Discovery board and the Ethernet interface, all working pretty well.
I developed a new board, using the DP83848 (which I think is used with the STM32F4 series on other boards?), the decision to change the PHY purely because of footprint issues, and the fact that we've used it before.
I was going over the PHY initializaton routine (HAL_ETH_WritePHYRegister and HAL_ETH_ReadPHYRegister), for the F7, and I just realised that the address for the PHY_SR (special register) is set at 0x10. However, the LAN8742 has the PHY_SR address (according to the datasheet) at 0x1F. The PHY_SR value of 0x10 is corresponding to the DP83848 chip.
I wonder if STM would be able to configure more known and tested PHY chips under STM32CubeMX -> ETH Configuration -> Advanced parameters -> PHY. You could keep a "user PHY" plus the correct values for DP83848 and LAN8742 when they are selected.
Just a suggestion.