The 'super glue and gaffer tape' view of the 230s is, I must say, a very lazy and boring stereotype. The real problems arose because:
a) The Ford Puma-based diesel genset package was not sufficiently proven, productionised or ruggedised before being deployed into passenger service (particularly true of the WMT units).
b) The build quality could have been a lot better in some areas. This was, again, particularly true of the WMT units which were delivered to Bletchley in 2018/19 in an unholy rush, indeed I think units 230003 and 230004 were actually incomplete when they were despatched! It would have been far better to keep the trains for longer at Long Marston and finish the build and testing properly rather than have them limp from Long Marston to Bletchley and then spend the next 12 months finishing them off and constantly battling with faults. But such was the desire to 'satisfy the customer'. Anyway, water under the bridge now.....
c) There were a number of silly design faults, most of which were eventually corrected through Engineering Changes.
But none of this was really because the trains were 'old tube trains', well atleast not a direct consequence of that. Remember that the trains were completely re-wired and internally re-fitted, apart from the bodyshells, bogies, some of the seats and the map panels - there wasn't much left of the original train. That which did remain from the old D78 didn't really cause any problems and many passengers had positive things to say about the experience of using the trains (when they were running.....)
Hopefully history will eventually prove that it wasn't such a lemon. The Island Line 484s are doing alright, the TfW units will get there and hopefully the GWR battery train will be the start of something a little more widespread on their branch lines. Three out of the four UK Vivarail 'fleets' eventually in service wouldn't be a bad result all things considered.