I have toured the Icon houses at Wolf Ranch. I went through their show house, but I also went to some of the houses under construction and examined them and talked to the workers a bit. You couldn't approach them them while the printer was running -- note also, they have a next-gen printer that looks more like a cement pumper crane arm, these were the previous ones.
Anyway the modification of them is addressed in some of the videos in the show house. Essentially you use a circular saw with masonry teeth to cut new holes, they provide shade-matching grout to fill in an old hole. It's less flexible than sheetrock but about what modifying a cinder block wall would be. Unlike most cinder block commercial buildings, the wiring is inside the wall and not in an exposed conduit, there might have been one exception in a bathroom or something.
Over all, to my non-professional opinion, it seemed more expensive than traditional "stick built" but also higher quality, probably worth it if you wanted a high quality structure.
I have also visited their site in South Austin on St. Elmo, and the small "tiny houses" they built in the Community First village for the ex-homeless, but I wasn't able to go inside those.
My overall impression is that it's a great technology that will be used for more and more structures. Thus far I think they have been too traditional in their floor plans, they have been focusing on showing that they can build real up-to-code houses that banks will accept as collateral. Hopefully with their new cheaper printer, maybe in some area outside of HOAs and zoning, they can starting making some more interesting houses -- like round towers Victorian style, for example.