The attack is known as the evil maid attack. It requires repeated access to the device. Basically if you can compromise the bootloader you can inject a keylogger to sniff out the encryption key the next time someone unlocks the device. This is what secure boot is meant to help protect against (though I believe that has also been compromised as well).

But realistically very few people need to worry about that type of attack. Encryption is good enough for most people. And if you don't have your system encrypted then it does not matter what bootloader you use as anyone can boot any live usb to read your data.

Full disk encryption is non standard and a PITA without a secure boot chain where the disk can be unlocked by the OS itself. If you have fun tinkering with your OS go nuts, but I want something that works every time even if install it for my mom. The current distro offerings aren't that.