Is your question what are some of the problems experienced by Alstom and Siemens?
Alstom - everything they have produced for the UK.
175s were awful until they had significant work on them. SWT almost returned the whole 458 fleet to the ROSCO, though they also have had significant work on them and are now exceptionally reliable. 390s were late and unreliable. 334s were late and unreliable. 180s were and still are very unreliable.
Siemens - 350s were late, 380s were late, bits fell off them and ScotRail refused to accept them and their initial reliability was very poor.
However, both of those companies seem to have accepted that their trains have had issues and are prepared to work on them - Alstom has recently brought one of their most experienced managers back over here to sort out any UK Issues, and from the work on the Pendolinos & Junipers, seems to be working very well. Whilst Siemens on the other hand seem very good at getting on with the job in hand, supplying rolling stock with their own depots and increasing the reliability very well. Bombardier on the other hand doesn't seem to particularly show much effort / care & attention towards the UK Market until someone such as LUL is prepared to go raise it in such a way (especially when it's affecting their service). All new rolling stock will have it's faults and niggles, but Bombardier seem to be taking on the role that Alstom once had 10 or so years ago.
My opinion of Bombardier rolling stock, poorer quality rolling stock compared to what's available / offer from other rolling stock producers (Siemens, Alstom, Rotem & CAF), with some designs which i would say are becoming rather outdated (well, they were inherited from ABB/BREL). And then there's issues such as the exhaust problem on the delayed the 172s, the issues with the 09 stock on the vic line with the over sensitive door sensors, and the constant issues with the S Stock.
Im sure others will have comments on the quality of Bombardier's stock too.