While I don't necessarily agree with adding things like Vault walls/doors, or some other specific things like that in the objects available for settlements, with the amount of things you see everywhere in the game that hasn't been implemented as available in the settlement system, it's hard not to assume that they went with 'mods will fix that' in mind when wrapping it up .....
The settler limit based on Charisma and the happiness sytem seem a bit odd as well ...
For the lack of options and the settlement-size limit, it actually bothers me to use mods, because even before starting to mess with it you know it will be annoying and it should have been made different in the first place ....
I mean I tried to work around Jamaica Plain the other day, spent a good 8+ hours on it just to get the layout right and 'secure' it as well as humanly possible, and just when I wanted to start to finish fence it up, and spruce it up a bit, I already was at the build limit ...
Then the castle, you start getting to an interesting place with your setup, you still have like 80% of the place available to build, but NOPE ....