What would I get by using MBR/BIOS-CSM?

Everyone is pointing to EFI but I wonder, would I win anything by staying on BIOS CSM Legacy Boot?

GPT provides possibility for larger disks and more “normal” partitions. UEFI is the more modern firmware. I run BIOS/MBR as tripple boot and had never any issue.

The simple answer is no - you won’t win anything.

GPT removes the limitations of 4 primary partitions per device.

If you are into encrypted root filesystems then GPT will allow for considerably faster decryption of root filesystem when using systemd-boot.

The systemd specification allows for discoverable partitions so for simple layouts fstab is no longer a requirement.

systemd-homed only works with GPT partition schema.