User blog comment:KnocknickKnock/Lucy's Unused Power!!!!!!!/@comment-1595565-20140713202332/@comment-7418318-20140724221611

There's a difference between Unison Raid and a combination attack.

What Lucy's Spirits did were combination attacks, since there already formed spells merged together, unlike a Unison Raid where the raw magic power of the mages is combined before any spells are cast, like the one between Juvia and Lucy you mentioned.

The only possible (and in my opinion very likely) exception to this was Gemini and Lucy casting Urano Metria together.

What Lucy and Yukino did when they closed the Eclipse Gate by summoning all twelve Zodiacs might also have been a Unison Raid, unless it was something else that is exclusive to Celestial Spirit Mages.

I know there's this stupid so called "Tripple Unison Raid" between Levy, Cana and Lisanna, which contradicts this explanation, but that was done by the anime team and Lisanna might have just called it a Unison Raid without it actually being one to intimidate or distract Kain. Combination attacks are done so often that I really can't imagine there being a monk who tried for his entire life to achieve it and fail, so I highly doubt that combination attacks and Unison Raids really are the same.