Gatlings and above are fine if you look at them on their own. But the face of battle has also changed in that era and above. There's less city engagements and more field battles because of the introduction of bombers and artillery. On the field, gatlings are just not good enough. Even with two range they can't do much against an infantry + artillery army, not to mention bombers.
With two range they would be much better. I'm not sure I agree with your reasoning about field engagements because extra range is a big plus for the xbow in the field. You can easily concentrate three on an enemy unit. In hilly terrain you get two hits on an advancing foot soldier before he gets one.
Gats suck even as a garrison unit. They are helpless to defend a city against frigates, and not much use against a cannon siege.