Massive generalisation from you there.
We have all 3 makes in our family.
Currently a Audi TTRS, c-class and 330d. All of them 2010 plates. All of them have been reliable, the merc has had a new electronics control unit though (not to be confused with a ecu) and that's it.
Test drove a4's, 3 series's and the c-class and tbh they are all of a muchness. Chose the BMW and merc purely on the basis that we have had alot of VAG's over the years compared to bmw's and mercs. The Audi interiors are still nicest, although the merc drive is the most comfortable
We are historically a Merc family, moving to Audi in the past 5 years, and it has been decided that we will be moving away for our next cars. This is not in my household, but we are a close family, uncles etc. inclusive.
Currently 4x Audi, 1x Merc. Plenty of VAG if everybody is included, and only Mk4 platform cars have been good to us.
The A4s are horrid to drive, so you made an excellent choice there. Two of my uncles own them.
The TTS is nice but unspectacular; not the best car to drive, even though PH were saying it was the best TT. Niggly problems like rattles, misfires, engine cut-out at 4k revs.
I like driving the Q7, though; the unreliability is the big problem there. Gearbox is shocking, but that has been fixed in the facelift, and the car (as Audi techs have agreed) was clearly rushed to market. The running gear was not suitable for a car which weighs 2.25t unladen in its lightest form. Suspension failure was common, tyre wear is ridiculous, engine is under huge stress, gearbox struggles, brakes wearing quickly.
Like I said, we used to buy Mercs only, and they have never let us down. We switch to Audi, and they have only been disappointing.
EDIT: Just realised I've veered massively off-topic. Apologies Seckles.
I'll summarise with this: Buy whatever excites you, because every car company has problems and you can't depend on anybody as much as you think you can. If you like the Alfa and want to take the plunge (and can afford the projected depreciation), then take it. I didn't regret it and I hope you don't either.