The real problem with playing charge+nova offensively in gold and silver is the lag.
I've only had one proper lag free silver game and I had no issue doing a charge+nova on banshees and brutes without getting instant killed. It's about how you dance around them as you charge and nova.
With lag however, a lot of the cues are not accurate, like, to you, you might be to the side of the banshee but to the game after compensating for lag, you're still in front of it. That's what gets you killed.
Then you have the stupid bug that gets you floating off the map enjoying sweeping panaromic views.
I've pretty much given up on vanguard coz of the lag issue.
That said, with a lag free game, it's hard to die if you know what you're doing. With a 200% cooldown (pistol), by the time you're done with a nova, biotic charge's ready to go and it brings your barriers back up again, rinse and repeat. Fantastic way to clean up the non-boss enemies. That's actually a strategy itself, allowing your team to concentrate on the boss while you mop up the kids.
If you want to risk charge+nova-ing bosses, only use it on banshees, brutes, atlas' and geth primes and charge from the sides, never the front. If you find yourself charging them from the front, roll away, preferably to the side.
After awhile you should be able to get into a rythm like:
Charge - roll - nova - repeat, or
Charge - nova - roll - repeat.
The only boss target your shouldn't charge are Phantoms. The other bosses are large and ponderous, needing time to turn to face you. Phantoms are agile and they can do an instant-kill from any direction.
If you still insist on charging them, then charge and roll back. I find that nova gives the phantom time to pick you up.
Super, super high risk of course, but in the right conditions, it's extremely fun and satisfying