Flashes for A6000

[pictureman]

Posted a flash question on lenses (not the best place) but got no replies so let's see if anyone has approached this situation. My Metz  flash units are very adjustable in that they can be set for ISO, A, M etc. I can set one on a tripod and a given Fstop, trigger it from a slave and it will illuminate the scene at the set Fstop.   What I've read is that when using these flashes on Sony cameras, the flash will trigger but can be used only on manual power settings. I'm not following this.  If, in fact, the flash unit is set on a given Fstop, why would it not illuminate the scene for the given Fstop, ISO  etc. when triggered by the Sony camera?  I don't own a Sony yet and this is my only hang up. I own three Metz units and don't want to invest in a flash for only the Sony.