Mixing RAID across disks, pros and cons

Many array, 2 disks on RAID 1, and rest on RAID 6. The full system is physically separated from the user data. One can expand user data partition without touching the operating system. Also it is easy to keep a spare system disk and save it off-site.

For servers with limited number of slots, one can mix RAID 1 (/boot) with RAID 6 (other partitions). This gives maximum disk redudency protection with available disk slots.

What RAID for swap?