How bad are bad sectors?

Depends on the file system format and the pace of physical degrading.

In case of traditional hard disk drives, since the Seagate Barracuda series arose circa ten years ago, I lost no disk because of bad sectors, but earlier than those disks I remember that already FAT32 and NTFS where enough good handling them.

So I would expect a very good support of bad sectors by FOSS file system formats and tools.