new map viewer doesn't really do related data yet. For example, can't use arcade on them in Forms.
End of the day, Experience Builder and New Map Builder are still missing major functionality and not ready. From my experience. Also, the Exp Builder interface is a step back from WebApp Builder as currently designed. Particularly for developers. I won't be using New Map Viewer or Exp Builder until maybe late next year, probably after UC release. I still can't believe Exp Builder's select widget can only select one feature at a time. It didn't even have Search or Print until last year I believe. I'm keeping our production stuff in WAB. I think it'll be a long time, probably a few more years, until WAB has any issues with webkit/chromium. Every year or so, I try Experience Builder and quickly realize... it's not ready.
WebApp Builder is still the champion. More features, easier to use interface for developers and end users, faster, better. There's nothing we need, WAB is vastly better at. Heck WAB even does 3d just fine, although we'll never need 3d 'digital twin' for anything. Same for classic map viewer. Esri team... I recall that "Express" mode for Exp Builder to make it more like WAB. I'm looking forward to experimenting with that. Once it can do related data, especially with batch editing; and it catches up with WAB in all widgets, I'll look. (Some key widgets in WAB are still missing. Near Me added this summer, added a lot, that was good. The Screening Widget was a dealbreaker, and that was just added, within Near Me. Now, for more.. Situational Awareness, Grid, Distance and Direction, Emergency Responder Guidebook and a few more, if they are still lacking. If it was in WAB there's good reasons. It was user driven.) All that to say, WebApp Builder was awesome. The whole philosophy, the team, the approach to it, great stuff. Same kudos for all the Field App teams btw. I am rooting for Exp Builder, I just want to offer insights from the field.
A related issue, no pun intended, is I am guessing new Map Viewer still has the maddening problem of attachments not working in joins. Which is crazy, because most often, attachments are in related tables. Which folks then join, to a point layer for example. So 1) we can see the related fields in the popup 2) it acts as "stacked" points, so each related record is a point on the map, for visualization and potential different symbology or def queries. Well... would be nice for the photo attachments in the popups from the related record. Alas.... Attachments don't work in joins. They don't come through. I suggested that years ago. That would be useful to everyone, right?
