About 10 years ago, I did a smooth ceiling job for a cousin of mine, and her father dropped in. He was a retired plasterer, an italian man in his 70's. He saw me scraping the ceiling, laughed at me and says "Why you no just coat over it?" So now, in most cases, I just bury the stuff in mud.
You do have to watch sometimes the moisture in the mud will cause the popcorn to let go from the ceiling. Which makes it a more costly, but cleaner wet removal system. Often though you'll be carving out a few bubbles, filling them in with more compound and with another coat everything should look great. In most cases though everything holds just fine.
Just to offer an entirely different angle on the problem. I've smoothed too many ceilings and walls over the years, and I've never thought about using a powered sander.