for the other candidates it would be interesting to know what drawbacks you see
I added some of the drawbacks. I do not see this thread as specific to the US, but perhaps useful for the countries which we approved several nominations (USA, Russia, China, and others).
As I hope is clear, this is very much my opinion and I could certainly be persuaded about some of the sites I listed in Tier 2. Some users may have had special experiences at certain sites or place particular value on the gap that site would fill.
2 (Approved) First Transcontinental Railroad (Golden Spike National Historical Park would be the centerpiece?)
2 (Approved) Fur Trading Centers of the Northwest (American components inferior to Canadian sites)
2 (Approved) Hoover Dam (marks the destruction of the Colorado River Delta)
2 (Approved) Mall of Washington DC (nationalistic and compromised by excessive war memorials)
I struggled with these 4 most, because their historical importance is without question. I generally have a harsh opinion about damming effects on the natural environment. Hoover Dam is an engineering feat and therefore, my struggle about its complex legacy. I think the Fur Trading Centers has the most potential, but on the American side the tangible remains are a logical extension, but certainly not of OUV alone. The transcontinental railroad is big in symbolic importance, there are even some preserved sections. However, it's not preserved in any complete level like other Historic railways and Promontory Point is hardly stunning.
The D.C. Mall is really special and I loved visiting these sites.
Yet, the war memorial boom (however tasteful) troubles me. These sites have been criticized for interrupting the historic character of the national mall and altering the un-interrupted views of its axis. There is also concerns that some of the locations have been specifically placed to disrupt protests. With more memorials likely on the way, the authenticity of the national mall dissolves further.
"But the 1982 dedication of the Vietnam Veterans Memorial launched a war-memorial-building boom. Monuments to those who served in Korea (1995) and World War II (2004) followed. Since then, the pace of one per decade has dramatically increased. Today, five war memorials are being planned for spots on or near the Mall. In addition to Desert Storm, they will commemorate World War I, the global war on terrorism, Native American veterans, and African Americans — both free and enslaved — who served in the American Revolution. On top of that, all three existing national memorials have been approved for expansions: Fundraising is underway for an underground education center beside the Vietnam memorial; the Korean War Veterans Memorial is slated to get its own Vietnam-style wall etched with the names of the more than 36,000 American military personnel who were killed; and, more modestly, a plaque with President Franklin D. Roosevelt's D-Day prayer is being designed for the National World War II Memorial." Washington Post