The table is somewhat misleading. The filesystem support for example refers to being able to boot a kernel that is on that FS from disk.
In most gummiboot setups, your kernel will be on the ESP partition along with everything else, so it doesn't matter what FS the root is.
The lack of support for MBR or BIOS doesn't matter much either, systemd-boot requires a 64bit System and most 64-bit systems that are still around and largely used (or actively sold) have a UEFI that supports GPT. If you absolutely need to, systemd-boot supports booting from a GPT that has a MBR wrapper.
So while the table looks like systemd-boot lacks support for a lot of things, the reality is that when you setup systemd-boot a lot of these columns simply don't matter
So does that mean that I can't use those filesystems for my /boot/efi partition and everything else is ok? Then it's really not that bad.
I haven't played around with other boot manager as you might be able to tell.
That's pretty much your limitation; your linux kernel + initramfs can't be on filesystems other than VFAT (ie, must be on the ESP but there is some ways you can have it work across disks).
Hence the table being slightly misleading.
For example, it also mentions that EFISTUB means you can't boot on btrfs and friends anymore. But it's the same limitation, initramfs and kernel need to be on the ESP, everything after that is up to the initramfs to bring up.
In most gummiboot setups, your kernel will be on the ESP partition along with everything else, so it doesn't matter what FS the root is.
The lack of support for MBR or BIOS doesn't matter much either, systemd-boot requires a 64bit System and most 64-bit systems that are still around and largely used (or actively sold) have a UEFI that supports GPT. If you absolutely need to, systemd-boot supports booting from a GPT that has a MBR wrapper.
So while the table looks like systemd-boot lacks support for a lot of things, the reality is that when you setup systemd-boot a lot of these columns simply don't matter