It's difficult going on number of occurences in a forum, because you need to weight it for the total number of cars out there. As an example, I haven't seen many posts about glitches with TVRs, but that wouldn't be sound evidence that they were more reliable than any of the other marques!
Also, with increasing use of shared platforms, it's quite hard to build up an accurate picture. For example, some Peugeots are buit in France, by Peugeot, but others are re-badged Toyotas and Mitsubishis. There are also "halfway" cars which might be Mitsubishis assembled in France...
Similarly, there should be absolutely no difference between (say) Renault Traffic and a Vauxhall Vivaro (both being identical vehicles, built from the same parts, by the same workforce, in the same factory, on the same production line)!
FInally, I've always felt (but never been able to know for certain) that the cost of the vehicle has something to do with it too. The owner of the top-of-the-range BMW or Merc is unlikely (at least in the couple of years that the JD Power survey covers), to skimp on servicing, use non-genuine parts, and keep it parked in the street. A much cheaper car could have to suffer all that, and it's the car that carries the reputation.