A practical issue with a circle is that it is not a circle until it is finished,
That's not the reason at all, AFAIK. The reasoning is, okay, we want people to be able to move from one place to some distance place in the city at the maximum comfortable speed, which is limited by G-forces. You have some guaranteed G-forces from first accelerating and then decelerating. But if it's linear, that's your only G forces. If it's curved, however, you also have radial G-forces.
The Line's train going from one end to the other (170km) nonstop is supposed to do it in 20 minutes, aka with a mean speed of ~510 kph. Let's say a peak of 800 kph. Now if we shape that 170km into a circle, that's 54km diameter, 27km radius. From the centripetal force formula a=v^2/r, that's 222,22...^2 / 27000 ~= 1,83 m/s^2, or a constant ~0,2g to the side. This is on top of the G-forces from your acceleration and deceleration. You can probably deal with ~0,2g in a train if everyone is seated without much discomfort, though it's double what's acceptable for standing passengers. But you can eliminate that if the city is linear (at the cost of increasing the mean distance that the average person has to travel to go from one arbitrary point in the city to another)
That's not to defend this concept. Because the city doesn't need to be 170km long; you can just made it more 2d and have the distances be vastly shorter (at the cost of just needing some extra lateral travel within the city). Honestly, if I were building a "designer" city from the ground up, I'd use a PRT (Personal Rapid Transit) system rather than trying to make it super-elongated.