My thoughts on WAB
As a developer I used WAB Developer Edition on two occasions in the last 3 years. Here are some of my thoughts.
1. Some important functions are missing in the out-of-box widgets. just think what you will do on the digital map: I will select features (by attribute or by interacting with the map), add / remove the new selection to / from my existing selection. But the Select Widget does none of this. and the Select widget only works for visible feature layers, no tricks can be done on the map service to make it seemingly selectable.
2. Actually the Query widget is more like a selection widget if a confusing and useless (most of time) temporary layer not being created for every query. Can't we just create a temporary memory layer which is managed by the application rather than by the user?? Most of user don't even know a layer is created for this operation. If I need my users to select on the map service (better performance in some cases, advanced cartography), I have to let them do a query first before they can select, what??
3. There are no Select By Attribute function. Instead, they provide two filter widgets , Filter and Group Filter. But sometimes I need simply select and highlight features rather than aggressively filter out other features. don't we do this all the time in the desktop GIS?
4. If I am not happy with the existing features / lack of features, I can always customize existing widgets, since I can write code and I have a WAB Developer Edition. right? I have to say I did get some good stuff done, but I don't like the experience. The main problem is the documentation for the JIMU architecture and its detailed API reference. It was so poor and out-dated that I had to read thousands of line of code to figure out how to add tens of lines of code of my own! I've been a developer on ESRI platform for quite long time, I feel the quality of the help document is not on par with other ESRI product for developers --- It lacks details. Developers need details, because their work are like under microscope.
5. I noticed there are many new widgets released since I first used it 3 years ago, but most of these are not useful for my projects. Let's go back to the basic widgets that everyone uses and bring back the consistent user experience that ESRI has established in their desktop products. how about starting with comprehensive selection function and doing stats/reports on arbitrary selections? and giving the developers stronger support by providing good documentation and examples if we are going to continue the Developer Edition.
I can't help thinking the Flex Viewer days, the application architecture was elegant and well-documented, so did the APIs, the spirit of the developer community were high... OK, have to get over it now 
I am not all negative about WAB, quite opposite actually, I like WAB in portal and ArcGIS Online, which provides great flexibility when sharing your maps. Just my expectation for the Developer Edition is much higher than that for the WAB online version.