At least in the US (except perhaps WN), the problem that I have is that there isn't necessarily a distinct line, it is more of a scrum or blob of people.
For DL/AA/AS the same Priority line is used for 2-4 different boarding groups, often times people in higher groups will camp out there and block those whose time it is for boarding. In these instances it's hard to know who is really group 1 or whatever and who is just hanging out.
UA is a little better with distinct Group 1 and Group 2 lines, though UA has so many preboarding groups (like 4 or 5) that that creates a different mess of people trying to get around the formed lines, plus there are so many in G1/G2 that the end of line can become their own scrum.
Some GA's will do a decent job of trying to manage the boarding area, but in other cases you may have a gate area boarding an A321 with enough space for a DC-9 worth of passengers so there is nowhere to go.