The ERM worked well in its first phase, from 1979-87 because the system was flexible, with eleven frequent realignments. The second phase between 1987-92 appeared to work well. There was only one realignment, when the lira moved to a narrow band. Yet all that happened was that problems built up below the surface. Nominal exchange rates did not change, but real rates moved badly out of line, providing the catalyst for the system's near collapse in September 1992.
Flexibility is important for any currency system.
The 1997 Asian currency crisis was the latest example of the clash between domestic and external needs. Previous experience of monetary unions in Europe is that they can last for some time, but ultimately EMU must become a political union to survive.