As a practice is the only criteria of truth, i made party of Mordred (SS), Hastur, Chronos, Zephyrus, Osiris and tried to check how all this things are working. I used water disaster raids as test subjects to prevent elemental advantage factor. Osiris and Zephyrus are both BL3.
Damage was for example Zephyrus 6.5-6.8k Osiris 5-5.4k
Burst - 16+k Zephyrus 24k Osiris
This means that with my weapons/eidolons damage difference is insignificant. Sure, with 100+% assault and Hraesvelgr the difference will be much more, so there should be no sence to use off-element himes somedays.
And i was wrong, Cronos debuff was realy b-type, this means i had second debuff from other source. This makes her less usefull, but she still deals nice damage. I will try setup with Zephyrus instead of Osiris for several days, maybe it will be batter. Caspiel is an option too, but she's only lvl20 so i will up her in reserve and use instead of Chronos to have 50% attack debuff.