I've carried a 7mm Mag since I was 14. I've taken it to Africa 5 times and have used it extensively in North America for game up to and including elk. The round is more than powerful enough. As others have noted, what you experienced was bullet failure, not cartridge/caliber failure. I've been using 160 gr Noslers in mine for years and it has always performed well.
As to the old "a bullet that exits wastes energy" stance, I say B.S. I want 2 holes if I can get them. 2 holes means more blood going out and more air going in through the holes. Any energy "wasted" isn't important if you have 2 holes in the animal.