Flat roofs are not a nightmare, trouble is caused by people doing nothing when they first notice a leak so it gets worse and they may dread it raining but that's because they chose to do nothing at the first sign of trouble. I have 2 areas of flat roof on my property, both are part of the original design but they weren't designed to last for ever and they haven't, both have been replaced and been problem free. I also have a conventional tile roof and two areas of that have needed attention, requiring the flashing to be replaced, tiles removed and replaced and the felt under the tiles being replaced. It was somewhat inconvenient but hardly a nightmare. I've lived here 20 years so expect to renew and replace parts of the property surely that's normal wear and tear. So as long as you are aware that certain parts of the building will need replacement over time I can't see a problem.