it completely depends on the specific model, you cant just say subaru is reliable, or audi is unreliable. both companies have their better and worse models. the 2.7t is a fucking time bomb of an engine, probably the most ridiculously expensive to maintain engine ive seen, yes, in terms of performance its fucking amazing, but unless you luck out and get the 1 in 10 that doesnt seem to have huge problems, you'll spend more on maintenance than you did on the car.
but if your talking about audi's old 20v engines, the c4 s4 and s6, a6, audi 90, etc.. those things are tanks, we had one guy with a 90 at something like 370,000 miles, stock engine, and he only came in for regular maintenance. all those older audis kill it in terms of reliability. BMW also has their good and bad models, e30's are beasts, e36's and e46's are pretty damn good. some subaru's will keep on ticking problem free, but you see some that are just complete lemons. and impreza's are not aging well, unless you can find a pretty new one, or an older one thats been babied, id pass, i just see so many of them with way too many problems.