With the (large) double sliding doors at each end open, it was sometimes a bit noisy (winter gales outside don't help!), but as soon as they were shut it was a lot quieter and more sheltered. It wasn't so much the roof that made the noise, just the wind hitting the partially open doors.
The roof might need cleaning every so often to allow the light through, especially if leaves/debris are left to settle and rot. Although even then it would be no worse than a traditional stable I guess!
We were only at this yard from September to late December due to their strange disinclination to actually feed my horse any hay
so I have no personal experience of summer/heat. My gut feeling is that it would have remained quite cool as the roof had ventilation bits, and there were the louvred windows at the back of each stable, plus the main barn doors could all be left slid open overnight.
@ fiesty filly - as far as I know, an American barn is a long central aisle with either stalls or stables running each side of it, and doors at both ends. The only 2 barns I have ever heard referred to as English barns were a large open plan barn that one of my old riding schools turned the horses out in for exercise in winter (no stables/stalls in it at all), and a barn at my current yard which is quite random in design - totally open at one end, and the stables are arranged in an L shape, but with a gap between the long and short side of the L. But there may be a better definition lurking out there
And an American barn in America may be different to an American barn in the UK!