Radiators 'failing' is not a problem on BMW vehicles, with the same engine. Forget 'thermal shock' and the like, this is down to radiators simply not being up to the job...
I would bet if a top quality aftermarket market alloy rad was fitted...there would be no concerns?
My mate with a TVR had an alloy rad fitted last year 5 litre engine car situated in a very hot environment - since fitting no problems at all (I think he said the rad is made by AlliRad) and the car is tracked.
Sorry Morgan but this is just down to poor quality radiators...someone (with an out of warranty CX car) is going to cook an engine anytime soon and a modern Plus Four or Six (BMW engine) is going to be 5 figures to replace...will Morgan stump up?
BTW - I have tried a Plus Six and would really like a (second-hand) one, however, reading all this does nothing for ones confidence and hurry to part with £50k, or more......!