The only combination I've had fun with is on a bowsorc. Bombard spam is something anyone can do, but combine it with streak and Negate and you've got yourself a legitimate group v group build. Incredibly annoying for the enemy and incredibly useful for your team. It's not overly difficult to play because you can just basically streak back to the safety of your group whenever. So you can be a little squishy if you want-- wearing plaguebreak or something-- or conversely you don't really NEED damage to pull this off so you can just tank up and try some ulti gen and/or cost reduction.
You are wise to worry about that. BUT. There is time between ulti dumps and it is important to strain the enemies resources in that lull. It creates confusion and delay in their actions. It helps separate them. It identifies people who are slow to break free or have no way to remove immobilizes.
Obviously before an ulti dump you would want to hold off. But the ulti-dump really isn't happening without your Negate anyway, so it's pretty much in your hands to control even if you aren't the crown.
Personally I like to drop the negate first and then bombard into it. A lot of people want to roll out of a negate anyway. It isn't wise but they do. Immobilize them inside of it and now even decent players might make that mistake. Only to get stunned by your teammates D-Leaps and whatnot. Which, when it works properly, you can choose to streak the half of the enemy team NOT in the negate instead, so that they can't immediately heal who's dying inside