I worked on the roof of an extension that had a lowered pitch to the tiled roof, The architect had specified Onduline as the roofing felt layer,. I'm sure it bought him a bit of kudos amongst his colleagues, but it was a bit of pig to make work.
The main problem came when trying to trim around the Veluxe windows. There were no proprietry fittings available to channel any water that got through to the Onduline, away from the top of the Veluxes to down the sides. I often wonder how well it has fared over the years since