Tfw have received the final class 769 yet . Not sure how they are behaving but I saw 5 of them out the other day
AIUI 769426 has not been commissioned. On another forum apparently three vehicles of the unit were delivered but with an unconverted driving car from a 319, possibly from 319374. These vehicles apparently returned to Wabtec with the 319 car moving to Long Marston.
Therefore TfW have eight of their nine 769s, all have been in service and they seem to be using four or five a day with six out on one day this week.
I don't really understand electric technology but I believe the class 319s were of a particular design where their chopper control makes them far more suitable for conversion than subsequent builds like class 321 onwards. It does seem to me that the conversion has left them at a disadvantage with straight diesel multiple units, but perhaps a considerably deeper level of conversion would resolve some of the issues. I believe that when a diesel engine needs restarting for example, it is not simply a case of "engine stop" then "engine start" buttons in the cab - a sequence needing to be followed so that the train understands it needs to shut down, then restarted. Older straight diesels tend to allow the driver to restart an engine without "killing" the train.
Regarding the TfW units, although they only work one route so far, it seems to be that same piece of railway where the failures tend to occur. One of the most bizarre reasons I have seen suggests some high voltage power lines across the route cause the shut downs. I struggle to believe it, but would never rule anything out in this day and age.
AIUI the TfW class 769s are redundant once the new trains are delivered - which is classes 197 230 231 756 and the tram trains, as well as the Mk4s of course. The final new 197s are apparently to replace the 158s on the Cambrian so it would theoretically be slightly sooner.