I'm not super experienced, but I've called a few bulls in with a bugle, and scared quite a few off as well. I have way more confidence bugling than I have cow calling. To me, it is by far the most fun elk strategy. You are literally trying to get an 800 pound testosterone-filled animal to come fight you. Can it get any better than that? Here are some things I have noticed:
You've got to be close. Inside that bull's comfort zone, so he sees you as a threat to steal his cows. Where I hunt, the woods are thick, this is typically less than 100 yards.
You have to fully commit to it, be aggressive, and live with the consequences. Your calling doesn't have to sound picture perfect like a calling competition 3-note bugle (most real bulls don't), but has to have that bad-ass attitude that challenges the bulls. Be loud, project the notes, and use aggressive grunts. If he bugles back, I almost always cut him off with a bigger bugle of my own, and that seems to be what triggers them to come in .
You are going to scare some bulls away and it may not entirely correlate to the size of the bull. Herd bulls are smart and know when they need to fight vs bail over the next ridge.
Realize that 95% of the time, the bull is going to try to circle downwind of you. I try to use terrain/topography and movement to counteract this, but they are going to beat you more often than not. It's by nature a very aggressive tactic that backfires more than it works. But when it works, man is it an adrenaline rush.