Taken as a silo, Onslaught isn't bad indeed and there is some use to the move that's a very small DPS gain.
However practically, Onslaught has so much potential to screw up your gauge alignment with Berserk/IR that you need to be careful about using it.
It's a good tool for dumping gauge before changing stance or during IR if you can't do the full hexaFC due to mechanics.
So it's not useless but it's not great either. it's much more valuable in defiance though.

Regarding stance swap, WAR is bad period. We have the biggest penalty for the least benefit and unlike other tanks, our penalty is extremely disruptive to our rotation.
It can be worked around with proper planning but there's nothing satisfying about it.
And having to go in defiance in an emergency situation is probably some of the worst feeling, similar to BLM loosing enochian...