Here are the two caveats I've found:
1) Size of volume limitations. It depends on CPU/RAM on the Filer, so different models do different things.
2) You really, really, want to be on 7.3.1 and any filers that replicate from it probably want to be on 7.3.1. 7.3.1 right now is new, so that scares some people.
here are the sizes:
Storage system model, Maximum size of volume without deduplication (TB), Maximum size of volume with deduplication (TB), Total data size of volume with deduplication (TB)
FAS2020 16 1 17
FAS2050 16 2 18
FAS3020 16 2 18
FAS3040 16 4 20
FAS3050 16 3 19
FAS3070 16 16 32
FAS3140 16 4 20
FAS3160 16 16 32
FAS3170 16 16 32
FAS6030 16 16 32
FAS6040 16 16 32
FAS6070 16 16 32
FAS6080 16 16 32
R200 16 4 20
FAS2020 16 1 17
FAS2050 16 2 18
FAS3020 16 2 18
FAS3040 16 4 20
FAS3050 16 3 19
FAS3070 16 16 32
FAS3140 16 4 20
FAS3160 16 16 32
FAS3170 16 16 32
FAS6030 16 16 32
FAS6040 16 16 32
FAS6070 16 16 32
FAS6080 16 16 32
R200 16 4 20