Do the data support that view? Not really I think.
Really? Let's revise the list (and these are only the ones that come to mind without doing some digging into delivery dates).
1. GWR Class 800/802 IETs- delayed and considerable teething problems when they were introduced. There was also the disastrous cost and schedule overrun for the GWML electrification project (even still it's not done yet!). Indeed that is one reason why the HSTs are late coming to ScotRail as the GWR HSTs could not be released in time.
2. LNER Class 801 Azumas- delayed, and then when they were delivered Network Rail found they couldn't even run north of Doncaster for a considerable length of time due to signal systems interference. They're also now causing problems north of Morpeth due to their higher power draw, which is why a disproportionately high number of Edinburgh services are still rostered for Class 91 and MkIV sets.
3. Other operators' Class 802 bimodes (e.g. Hull Trains)- delayed. They had to hire in HSTs to cover the shortfall.
4. TPE Mk 5 sets- considerable delays and they are now suffering from poor availability. No complaints about the locomotives at least...
5. TPE bimodes- delayed, poor availability and oh btw they have to run on snail-slow diesel power north of Morpeth due to capacity issues drawing power from the 25kV OHL.
6. Anglia class 755 bimodes- delayed.
7. ScotRail Class 385 EMUs- manufacturing schedule delays and then an expensive and lengthy post-delivery delay due to to retrofitting new windscreens that didn't form a "ghost" image of each signal in the cab when the train was approaching.
8. ScotRail Class 380s- delayed (can't remember offhand what the issues were here but it was not pretty, I remember that!)
So yes the data does support my observation. It doesn't matter whether the stock is new or 40 years old; when it's introduced onto new routes there are invariably teething problems and it takes time and teamwork to fix them, not to mention hiring in other trains to cover the shortfall until the problems are resolved.