There's only two exceptions I know of. The first is if a model in the unit has Split Fire, meaning you can target a second unit during the shooting phase. You may then assault that second unit.
The other is the special rule for Corsairs under the 'Reaper of the Outer Dark' Prince specialisation. That states that you may give any unit Rage for free, but that unit must charge any enemy unit within 8". That means you can do cheeky movement shenanigans lighting one unit up with pistols, Reckless Abandon towards another unit and then be forced to charge it in the assault phase.
The only thing I'm not sure with about that rule is how it affects ICs attached to non-Corsair units. If your Prince is attached to a unit of Wracks who shoot up something with an Ossefactor, but are within 8" of another enemy unit, do the Wracks have to assault the other unit, does just the Prince have to assault the other unit, or can they not assault?