SMN is broken purely because it has sustain, burst and recovery. It's like a hybrid class which can fill all roles except pure healing (Actually to some extend they can if they Materia Mind and are able to heal upwards of 700-800 per Physicks). FotM class is one thing, but totally broken FoTM is another thing. Basically any FoTM class will have one most important characteristic - Ability to Global someone with relative ease.

And as it currently stands. SMNs are capable of just that and MRDs, though MRDs need CDs to execute and can be kited whereas SMNs are free to do. Take for example if say BLM's Scathe actually has a chance to proc Firestarter has 120 Potency and 20% chance of dealing double damage. It too can be a FoTM class, since the BLM will just scathe to a Firestarter, Scathe more to save another firestarter, then get a Double Scathe Crit follow it with Firestarted in AF3 Quickcast Flare.