Deglazing is pretty hard on the seasoning of the pan (basically polymerized fats from cooking), which means your stuff will probably stick next time you cook with it.

I actually use exactly the method above for caramelizing onions, and I use either an enamelled cast iron pan or a tri-ply stainless/aluminum pan. Both work well, but I think the results seem a little better with the cast iron. That may just be because I can go straight into making onion soup in the same pot, so every bit of the sticky onion stuff makes it into the soup.