Yes, I definitely see value of comments in a situation like flex fields because that's something that may not be straight forward or clearly apparent!
Legacy tables or fields also make sense because the code will remain the same and thus lead to no comment rot.
As long as the comments stay relevant and cause no extra confusion, then it is useful! I suppose it is also situation dependant as well as the preference of the development team at the time.
I believe that documenting anything with comments (be it code or a PostgreSQL database) should only be done for more complex pieces of code or to elaborate on something that could be misinterpreted.
Misguiding or incorrect comments caused by "Comment Rot" will actually cause more harm than having no comments at all. In fact, your code should be self explanatory (if you do it correctly) in terms of what it is doing. Adding extra comments to elaborate on "why" something is being done may be more useful - i.e. When the semantics of the code are not as easily deduced from the syntax.
However, I do find documenting database tables interesting and will consider doing it in the future under very specific circumstances. Thanks for sharing!
I think that this is such a tough question to definitively answer because the measure of difficulty of a problem is all relative to the particular point in time.
I am currently working on my thesis in artificial intelligence which to me seems tough because I have never written a thesis before. However, at work, I am dealing with technical software engineering problems that will seem easy after I have solved them.
My first industry project involved creating a generic form builder which could ultimately be used as a survey tool to draw statistics from. This seemed extremely challenging at the time, but now that all of the design decisions have been made, and complexities solved, I could redo it pretty easily (even though we shouldn't recreate the wheel)
Legacy tables or fields also make sense because the code will remain the same and thus lead to no comment rot.
As long as the comments stay relevant and cause no extra confusion, then it is useful! I suppose it is also situation dependant as well as the preference of the development team at the time.