I found that without the speed sensor, the GPS would momentarily lose signal when I was standing and climbing, with my body obscuring the Edge 500's line of sight to the sky. I would get random auto-pauses and speed drops/spikes.

I've also found that outdoors, the Garmin gets confused with the speed sensor on the rear wheel, so I have to run it on the front hub. But for whatever reasons, it has zero issues indoors on the trainer.