You need to get hundreds, if not thousands of tonnes of mass, high into the stratosphere. That would take a VERY large balloon.

Then you have to drop the rocket and fire it off without bursting the balloon (unless you don't mind all those tonnes of balloon and launch-control equipment plummetting to earth). - sure, you could parachute the stuff back down, trailing shreds of the giant balloon behind you. Maybe you could even design a balloon that "unzipped" into a giant parachute?

AND, you have to cope with the sudden unloading, and lose those thousands of tonnes of bouyancy.

So there's going to be a LOT of helium vented into the upper atmosphere. Any takers on this being an environmental problem, somehow?

Hydrogen would be easier. You could burn it away, and only "pollute" the stratosphere with water and heat and some trace emissions. That *might* be easier.