I can't say I've ever been on a brand new Siemens train, so accept that I might have some rose tinted glasses when it comes to thinking of them as reliable, but I can remember a virtually brand-new 377/5 on FCC where the disabled toilet door opened, creaked, and stuck. And they were delivered very late too.
I can remember the door problems on the Victoria line trains (ironic given the old stock had such bad problems and everyone would have been dying to get rid of them to get rid of the problem, not get it happen just as bad) and the many times the trains were delayed - although fortunately, never on a train I was on, taken out of service. But certainly delayed - many times.
I can remember seeing on all the newly delivered trains the rubber seals around panels hanging out (still like this today) and all dirty from what has obviously been an awful lot of panels removed for some reason - and put back badly, by people with dirty hands. Any ideas why so many panels have been removed? My guess has to be to fix/replace something.
And then the dirt/dust throughout the train, when I believe the filters had to be removed because it was messing up the ventilation.
Then there were big problems on the S8s, including a clip of a TV documentary on the tube that showed the anger and frustration at TfL's end, and some problems on the 172s.
I'm fairly sure the 379s also had a fair few issues.. so that leaves the 378s - which I can't think of any problems? Maybe they got lucky!
And isn't there some fuss over a resignalling project that involves Bombardier in some way, where the contractor has walked away?
Let the media praise the support of a 'UK firm', but personally I have much more faith in the trains coming to Thameslink than Crossrail, and my concerns for Thameslink will be mostly down to the other infrastructure (track, signalling, overhead wires etc).