My gf and I spent 2 months living with my mother and her bf while I was waiting to close on my first house and doing the most intrusive renovations. It wasn't that big a deal, and nobody felt rushed to get into the new house asap. Obviously, that's a relatively short term compared to a situation like Zeig's relative.
I'm still curious how many of the people who see this sort of arrangement negatively have good relationships with their parents. I think that's important, whether or not you live together.
I think this depends really heavily on the phase of life and where you are living. We lived with my parents for about a year, 10ish years ago when the boat we were going to live on ended up being a clusterfuck that needed new rudder, wiring, plumbing, etc.
I've always had a good relationship with them, although my mom had early symptoms of FTD (unknown at the time) that resulted in some weird behaviors, but even aside from that it was strained. In retrospect I think it had to do mostly with the size/layout of the house we lived in, it was a single story house with all the bedrooms on one side. I also worked from home and they were retired so we were there all the time. We had a good relationship but living in such close proximity to two other adults was really challenging at that phase of life.
In contrast, my dad lives with me now while his house is being rebuilt and it's a non issue. He has a room on a largely unused floor, can come and go as he wants, but we're also older and things are less contentious.
I usually respond negatively to it despite us having a good relationship, and yea it wasn't pleasant, but thinking about it more, I believe that probably had more to do with the circumstances surrounding it than the fact it was my parents. I'd be curious, major relationships issues aside, if there aren't a subset of people who respond negatively purely because it's so many adults in close proximity as opposed to being purely relationship challenges
