Not really the case.
Pharah can be stunned out of ult.
Junk (before finishing riptire animation)
Rein (very small window, but it is possible)
Lucio (same as Rein)
Also @mizx covered a few others.
It’s just that the stun = ult cancellation is new to the support roster.
My problem with it is like what you said, being lackluster. I’d rather have the beam be a switch, between healing and damaging, using right/leftclick to change the beam type. The whole dual beam just means that the healing and damage is watered down.
That, or I’d like a bigger beam animation so this way it has more forgiving LoS for the meh damage/heals.