Reliability has several aspects.
1) Does the car need frequent repairs?
2) Are the repairs debilitating ( ie are you down and out , without the car for extended periods)
3) Are the repairs expensive ( parts and labor)
4) Are parts and mechanics hard to find? Diy?
On count 1, Mercedes is not as good as the leading Asian brands. ( as per my own anecdotal evidence and Consumer reports)
2) Not too horrible esp. on older models.
3) Horrible. Dealer service is super expensive both for parts and labor.
4) Bad. Parts are harder to find and sign ificantly more expensive than the common Asian brands
e.g.: Water pump for the cla is $250-$300. (Typical for other cars: under $100).
IMO strictly on a rational basis, the neither of the German Luxury brands make much economic sense for someone who
cares about maintanance cost. For a fairly well off, person, wanting to enjoy the prestige and driving fun, who is aware of and willing to pay the premium, it may make sense.