Eh they're weak if you just use them with no combo or weakness hit, otherwise they can be fairly strong. It's just hard to maintain 3 BG when I'm fusing and stuff all the time.
You can only use them in a combo .
And I generally do them in a combo but even then, they aren't that strong. (the ones you get
after the trials seem strong though based on how strong the fire one is. Not got the others yet
My favourite was being able to pull off a 96 or 97 combo or something and the MA (Soreys default MA) bringing it to a nice even 100 hit combo and finished the fight. That was awesome.
also yeah it can be hard to maintain 3BG when fusing for boss fights and stuff. Especially earlier in the game where I decided to defuse to MA and forgot to take into account the BG split when you defuse >.>
edit: also I found it totally awesome when Edna used her MA (as the AI) and finished the fight. Then I curse at the next fight where Mikleo used his at the start of the fight which was a bit of a waste but oh well >.>