Paul Ingraham

They have been promising export preview since the earliest days, their own roadmap, not just wishful thinking on my part.  For users like me (webdev), it was "late" a year or two ago, by any reasonable definition of the word "late."  And now?  Now it's still missing from the betas for 1.7, in late 2018.
"Betrayal" is emotional language, maybe inappropriate for this purpose... but darned if I do not feel it!  I got on the Affinity Bandwagon early, to support it, long before the feature set was really ready to replace PS or Fireworks.  Export preview was a critical missing piece from the earliest days, and I stayed on the bandwagon BECAUSE export preview and some other things were on the roadmap.  I continued to use (and evangelize) the apps because of that promise.  I gave Affinity my money and a LOT of my time, not for features I HOPED they would build, but for features they actually promised.
And now here we are and still no export preview.  I give up.  I am getting off the bandwagon.  I will now go crawling back to Adobe.  Instead of telling colleagues how cool the Affinity apps are, I will now be telling them how disappointed I am. So yeah… "betrayed" doesn't actually feel too hyperbolic.  No doubt Affinity Photo is a replacement for Photoshop for some people, but it really is not for webdevs, and I now suspect that it never will be.

Hi Paul Ingraham,
Welcome to Affinity Forums :)
The Export Persona will be improved with more export options and previews later. There's already a few threads requesting it so you're not alone on this one.
Just give devs some time. There's several features being worked...