We have the benefit of hindsight. At each stage of it's development there have been minor issues, but each in their own right, not enough to stop the programme. Opinions differ as to if and when to give up, each coloured by significant issues such as sunk cost, remaining budget, estimated cost at completion and of course alternative stock availability. In the case of the 769s:
they are at last working resonably well as bimodes in Lancashire on Manchester - Southport services
they are also working acceptably well in service on the Penarth - Rhymney route
they have been tested with relatively few issues between Reading and Redhill
So in practice, given that the remaining issues on the GWR units concern cab ergonomics and other matters that are more to do with general shortfalls rather than their specific bimode characteristics, it would seem that with no alternative available, they will continue for the forseeable future.