I think this is a case of a load of old blokes on a forum not liking new things as they're different to what they grew up with. I personally think BMWs look OK on the whole, but yes they're no E39 or E46 which were IMO the last of the really good looking BMWs. However, it's just my opinion and that's all it is... many people still want to buy them so I have to accept there is no right and wrong. I also don't necessarily think the build quality is any worse when you consider the extra equipment and features people now expect... basically there is a heck of a lot more to go wrong. I have a 57 plate E70 X5 (out of necessity), but it has been uber reliable and has just gone over 100K miles, my mate has a 2015 X5 and has over 120K on his... very few issues on that one too.
Mercs on the other hand I do think are not the cars they were. I own an R230 SL but it's bloody fragile.... costs a fortune to keep in top fettle. Still as it was (IMO) the only decent looking car Merc have made in the last 30 years I don't mind... it's almost like the Jaguar designers snuck in to Merc and designed them a decent looking motor then left
All in my opinion of course as I realise Merc are phenomenally successful right now.
It's hard becoming a curmudgeon