I think that petty much all the VAG group of cars are well built and reliable. VW, Audi, Seat and even Skoda to name four.
Some of the more recent Seat models have been designed by former Alfa Romeo designers, as well as being influenced by Audi Concept designs.
Seat use the same parts used in a lot of VWs. Ibiza has the same chassis and engines as the Polo, the Leon the same as the Golf. Often the Seat counterpart to the VW outperforms the VW one. Of course the VW has better interior etc.
I think the main differences between them, they are trying to make Seat more of a sportier fun brand to appeal to younger people, VW more practical, and maybe family or young professional orientated. Skoda for families on a budget and Audi more classier, prestigious/ luxurious.
I believe the cars they have made in places like Mexico and Argentina are completely different from what they make in Europe and not as good in terms of quality.
Maybe someone else with more knowledge would like to input on this? I am not any sort of expert when it comes to cars and don't work for any of the VW/Audi group of companies.
John