We used an ergo last year, just a few times when our 8-month-old was napping, and it was mostly okay, except for one bad experience. You are unfortunately at the mercy of CMs, and you have no idea what they might say right before you board a ride.
The good: no problem on Winnie the Pooh, even with a lap bar. Also fine on Triceratops Spin. There were a couple of other rides that I don't remember, and of course it was also no problem during character meets - he slept right through Baloo and Louie while his older brother had a great time.
The bad: at the very front of the line on Toy Story Midway Mania (at the bottom of the stairs, right when everyone is in a rush to board), the CM made my wife wake up our infant and seat him next to her. The CM insisted he couldn't even be on her lap, he had to be out of the ergo and sit by himself. As you would imagine (after being suddenly awakened into a manic environment), he screamed for the first 80% of the ride, and neither of them could enjoy the ride at all. Our older son and I were also distracted by the screaming, so none of us enjoyed it - and I imagine other nearby guests were also not exactly hitting their high scores. I can't imagine what the point of it was, from the CM's perspective. Our infant would have easily slept through TSMM, and we all could have enjoyed the ride.
We had an old version of the ergo, where the baby can only face in.