Some of the (real world) feedback I got from the web apps that failed to get off the ground were due mainly to our customers complaining that:
* pages were taking too long to reload.
* not enough fields on a particular data table to store information in
* missing API
* pages that would refresh in entirety instead of just updating the changed portions
Most customers didn't stick around to wait for us to address or fix the issues - there were plenty of other competitive products that fitted them better that they could start using that same day.
A couple of those sites also got negative feedback on Reddit or HN because I hosted the front end website on a $5 VPS server and when the site suffered the inevitable 'hug of death' from posting to these sites, they immediately went down due to inability to scale under the sudden deluge of visitors, and I received uncomplimentary feedback on that (where people actually elected to post feedback rather than just close the browser tab and completely forget and move on from my app).
Yes, over thinking scaling and design is bad, but putting it aside as a 'totally not important now' factor is also just as bad, if not worse, in my 'lived it' experience.