Pro:

• do most of what needed two phones on a single phone
• avoids charging, carrying, and maintaining two separate phones
• I'd personally put a lot more here if dual-booting Android and Linux

Con:

• hard to find a phone that supports it
• the need to unlock bootloader could still break integrity checks despite using a stock ROM
• IMEI still shared between ROMs, most of the isolation is already achievable through user profiles
• have to reboot to use anything on the other ROM