The biggest issue with leaking flat roofs in Britain is not the system applied as a waterproof seal, but finding out what caused it to leak.
Most flat roofs have inadequate fall and are built with substandard materials, due to these issues flat roofs begin to sag allowing water to pond, which will then overwhelm any joints or seams and flashings of the waterproof covering.
Flat roofs should have a decent fall to allow rainwater to run off freely and have a proper 18 mm deck of plywood or OSB board.
If your have a shallow pitch flat roof or a flat roof constructed with substandard material it will leak eventually no matter which material it is covered with.
For me my preference for flat roofs is the latest high tensile torch on felts
which provide excellent adhesion and have a 50 year life.
Rubber or fibreglass options may or may not last this long only time will tell.