Rumor is it is impossible to boot an operating system from the software RAID array.
This is not exactly true. All the widespread boot loaders, namely ones from Windows NT/2000 and so on, Linux LILO, and GRUB, would successfully use a RAID 1.
It should be noted that further steps should be undertaken for bootable mirror. Since the mirroring does not cover the Master Boot Record, you need to copy the MBR sector manually to the shadow drive in the mirror. Otherwise when the boot drive crashes, you are left with an unbootable system.
Surely there will be no loss of data – all you need is to just mount the drive into a known good system and read it, but you get no automatic recovery if the hard disk dies, requiring a reboot.