Basically these are the pros and cons
Pros:
- Reduce nitrates naturally
- Creates a breeding place for pods
- Increases the dversity of your tank
- Increases total water volume - Dilution is the solution to pollution
- Can take up many interesting forms (DSB, Algae scrubber, Cryptic, Hospital, Hair Algae(yes hair algae), mangrove, plenum and traditional)
- Can be used to keep seahorses (Pod population will be lower)
Cons:
- Algae may crash or go asexual
- Disaster brought upon beginners putting sand shifters in a DSB
- Inavailability of "recharge kits"
- Some are difficult to set up (eg. hair Algae)
- Run the risk of algae migrating to main tank
- Difficult for some to understand the dynamics