I just think of them as higher explosive yield photon torpedoes, like quantum torpedoes were an improvement over photon torpedoes, that can pierce the shields of starships. Their use isn't banned by treaty and they don't tear subspace like So'na weapons. Their use doesn't cause toxic radiation or other toxic after effects. I seem them being rolled out to the Starfleet.
They can't be just higher-yield bombs, because if they were, then all you'd need is a big enough spread of torpedoes. Besides, the label "transphasic" would have no meaning in that case.
What Dave and I worked out between us in the books is that the torpedoes employ a "subspace compression pulse" that exists in multiple "phases" (hence "transphasic"), like how Geordi and Ro were "out of phase" in "The Next Phase" or how the two differently phased copies of
Voyager could coexist in the same space in "Deadlock." So if the Borg shield against one phase, the explosion still gets through in other phases.
So it's really not so much about being more powerful as it is about being more effective at penetrating defenses.
I believe that hostile powers to the Federation will develop an effective defense against them and their use as "super weapons" will be short lived.
Maybe. Indeed, one would think that the "multiphasic shield" seen in "Investigations" and "Descent" would already be on the right track.