When I first got into geoFlex dev, I jumped right in and started making really cool UX centered "apps". The learning curve was very reasonable. It included everything contained within the Flash Builder IDE: MXML, Actionscript, esri API, Flash Builder (Eclipse) idiosyncrasies. This allowed for a very standard, well understood development environment, be it esri GIS or not. You could go to a Flex Dev session and you were 100% guaranteed that everyone is on the same page. I feel that Flex geo-dev really allowed the developer to concentrate on crafting an end product that hits the nail on the geo-UX head and not get bogged down in the mechanics of the dev platform.
With pure web dev, I'm finding things much more challenging for the following reasons, all of which have their own discrete learning curves: HTML, CSS, JS, Dojo, jQuery, AngularJS, editor or IDE of choice (and there are a lot), responsive design patterns, task automation, testing frameworks. There is little standard here and when you sit down with esri Dev's (or even non esri devs), there is no "common ground". This dev does things one way and that dev does it another. Truly, to be very productive at this, one just needs to totally geek out on "web dev" and get neck deep into it and just love it for what it is. I haven't found a sweet spot yet and this after working with html/css/js for a few months now.
I remember during the 2008 UC, I was able to create unique Flex based web map apps from my hotel room, show them off to my manager and have very positive response. Within a year, things were really humming along. That isn't the case now with HTML/CSS/JS and I haven't put my finger on the reason. Maybe I'm just getting old and grumpy. I'd like to think that's not the case.
