I've been working on a new product to build websites, and I'm going to start the second iteration of development. Your answer can help a lot.
I'm mostly interested in targeting the market for simple website builders for the non-tech-savvy. Hence the mention of these two services that I think own a large part of this market.
For example, my son was required to use one of these builders to make a site for his English class over the weekend. He wanted to connect up a simple link in the homepage to an about me page. Simple, in a static website this would be an <a> with appropriate href and such. But with a site builder you had to know which box to go to. You had to have already made the about me page so that the site builder knew how to link up. Etc etc etc. A task that takes me 3 seconds took 10 minutes.
Additionally, as someone who works for companies for a living, I get calls from potential clients that say "I want my Squarespace to do X, Y, and Z, and Squarespace doesn't let me do it. Can you make it do it?" Well, no. I can build you a site that does that, but I can't make Squarespace do something it doesn't do.
These site builders aren't bad at all. Non-technical people use them successfully all of the time. But I am definitely not your target audience.
I'm thinking if my builder can make a software engineer build website quicker with a high level of abstraction that's exclusively mutual with others being able to do so. I've been researching intuitive interfaces, as my hypothesis is that the interface is the bottleneck. Thank you again for your comment, it is helpful!