First impressions:
Eradication performs slightly better in ST. Though first I was a little confused that I have to track the debuff and plan the rotation accordingly, whereas having T19 2p bonus leaves temptation to just spam Chaos Bolt.
ELT isn't far behind in ST, and for M+ it is a more preferable option since it buffs all outcoming damage. Pre-cast ELT, then Cata -> Rof -> CDF = HELL.
Cata vs FnB, well it depends on dungeon and pace. The only thing that kept me from taking Cata before 7.2.5 was its CD. Now its lowered but so is the instant damage it does. Moreover sometimes packs die so quickly that RoF and Incinerates are the only options. 3+targeted FnB makes shardbits accumulate really fast.
Last tier - CDF. Strong in ST and fairly strong in burst AOE. Soul Conduit, well... I got tired of RNG. And Havok became a headache IMO. Incorrect timing will result in a huge dps loss and taking Wreak Havok hardly improves the situation - cd is still too long. So leave it default and just make sure you use it correctly.
Sure there are (and will be) council fights where you can take full advantage of it.
All in all, new (old) shardbits system is pleasing. Doomshards addon was updated to 7.2.5 and is a musthave.