I have lived in Harlingen, McAllen, and Edinburg. Harlingen is definitely different than other towns in the valley. I think the reason is that (I know it was a long time ago) Harlingen used to have an airbase. Many folks in the airforce retired in Harlingen. Even though that was a long time ago, the retirees still have an impact on Harlingen. After the base closed, the military tradition continued with the Marine Military Academy, which brought in retired marines. (I taught at MMA for a couple of years.)
This retired military culture has made Harlingen a very conservative town. For years the town (city) would not support the schools and defeated bond issue after bond issue. (Retired folks don't have kids in school.)Whether you think this is good or bad, it is a fact that most of the valley is very familiar with. Harlingen and Weslaco have tried mightily to copy the merchantile successes of McAllen. Harlingen has had some success but it does not have a bridge. Weslaco has not succeeded and needs to really make some changes fast or be left in the dust by Mercedes and it's outlet mall.
In the last few years Harlingen has done some good things. They have a mall, and they have made good use of the old AFB for several things. The schools have dramatically improved and are on the same level with better schools in the valley. The Baptist hospital is as good as or better than the hospitals in McAllen. All that said, Harlingen seems to be a clique-ish town (probably a hold over from the old military days).