I think it very much depends on which model you may own. The higher the volume of the model, the more reliable they seem to be, which makes sense. The R8 was pretty problematic, hideously complex and expensive to fix.. it was an example of a low volume car that had too many quirks to be reliable, and I ultimately hated it. I also had an A8, that was very low volume, overly complicated, and broke all the time.
Gone On numerous VWs, and they are mostly reliable, but can be hit or miss. VW designs for cost, as opposed to their German competitors which design for some maligned version of “luxury/sportiness/complexity”. Designing for cost is much harder to achieve real reliability with unfortunately, and VW has had its share of issues on many of its car models. I’ve had good luck with certain VW models (Golf), but lots of problems with others (Beetle). Oddly, while VW and Audi share many parts and platforms, the Audi variants of some of their cars are more thoroughly refined, especially with their higher volume cars (A3/4/6)
BMW is also hit and miss. They try to be too fancy for their own good sometimes. There have been disastrous forays into in-car electronics that nearly cripple the car, horrible transmissions (SMG anyone?), bizarre material choices that led to major engine problems, and many other places where they tried too hard to be special. I only owned older BMWs and haven’t had one in a few years, mostly because their cost is so absurd for a comparably equipped car, that I usually left the showroom shaking my head. That, plus their nasty choice for in-car interior materials always turned me off.
Mercedes is a bit different, in that they tend to live in the past more than VW/Audi and BMW. They are slower to try new things, unless they’re not (huh?). They tend to stick to tried and true more than their direct competitors for many things in their cars, which means a more conservative design for some subsystems. It also means that they tend to seem outdated sometimes also (dash controls are a classic example).
Emotionally, I’d say my brain seems to think that I’ve had the best reliability with Mercedes, then Audi, then VW, then BMW. I have no statistics to back this up other than some recollections of how often I’ve had them in for repair or had to fix them myself. I never counted, but if I think of how well I know the dealerships/repair shops, then that order is about right by my flawed middle aged memory.
Now, on to my weird statement about Mercedes. When they do try new things, they do seem to get them right more often than wrong on their first try. Super high HP motors, oddball transmission designs, and other things seem to fall their way more than with their competitors. Their specials or low volume cars I’ve owned seem to be less prone to failure than the other guys. No idea why, maybe they test more, maybe they take longer to come to market. I don’t know precisely.