So I've been following Spreadsheet for a while as well as their competitors.

In the spreadsheet-backed dynamic website space they are unique because of the search functionality (some of their competitors don't have it) and because they use google sheets instead of airtable. (airtable limits api calls from a certain point onward).

I do have some requirements that they haven't and that I'd like to see implemented:
1. user accounts and limiting blocks/pages to certain users;
2. having the ability to add more blocks in order to create a better looking page;
3. allowing a tree-like structure where we could have any page flow we want.

Because of this I'd say Spreadsimple still has a lot to grow, but then again you don't see any LTD in this space, and specially not with these generous limits.