So when people talk about 240/260z being not entirely comfortable
Not entirely sure which people have said that as I have never heard it yet.
What constitutes comfortable for one man is not the same for another - in fact what is the definition of comfort?
If you mean suspension then it can vary wildly from one S30 (fairlady/240/260/280) to another depending on who the previous owner was and how they set it up, which market the car came from etc.
If you mean engine / exhaust noise then see above.
In very simple terms, think of it this way, early S30s like a 240 felt more sports car and the engines really came to life over 4k rpm so you had to work them. The 260 was more GT with a better torque spread (mine used to pull from 1100rpm in 5th), a taller final drive and better seats / interior. 280z was even more GT and had more creature comforts.
With Stock suspension, bushes, tyres / wheels - all will feel soft and wallowy compared with a modern sports car like an MR2, S2000 etc. In fact the only time I had had enough of my S2K was a 4 hour drive to wales, but these are sports cars not mile munchers.
My 260 fatigues me on longer journeys as I have a noisy exhaust and a modified engine making a glorious noise + I like to set shocks up for handling. Oh and there is the fact that my car eggs me on and makes me drive hard, officer.
Sound proofing is nowhere near modern standards but I added some myself which takes out a huge lot of wind and road roar.
The seats on my late 260 are very comfortable and provide good support. Some complain that they sit “on” their seats rather than “in” them, not an issue in mine.
North American market cars are softer in stock form than RoW cars.
My advice, don’t believe everything you read, wait for covid to pass and get rides in members’ cars to get an idea of how varied they can feel. Unless you are comparing totally stock condition cars, every car is going to feel different and this is a difficult question to answer over tinter!