In any of my carriers it's taken some practice, but nursing works well in any of them!
In the ring-sling, she preferred to be held in the snuggle hold (I think that's what it's called? her facing in, upright, with her tummy against me). To nurse, I just loosened the ring and she was at the perfect level. Now she also likes to lie down in cradle hold, but like a previous poster, she detested it until I randomly tried it just recently.
The ergo works great and in a similar way. I just loosen the straps and she's at the perfect height.
My pocket carrier is mostly used at home because while it's convenient to just stick her in without making any adjustments, it's harder for me to nurse discreetly. But at home I don't mind who I'm flashing ;-)
Lastly, to make nursing in a carrier most convenient, I found that I need to plan my outfit a little bit. I have a few nursing shirts with hidden openings that are great (especially the ones that have a band under the bust area since it's easier to lift up). I also have a few nursing tanks that allow me to then lift up my shirt without exposing my stretchmark-riddled belly to the world.
During my pregnancy I kept an eye out for any and all inexpensive carriers and just got them all. I actually use all three, but wound up not using our older model baby bjorn at all. Anyway, it's been great having a choice. All the best!