We actually took one about 20 years ago that started in Rio, went across the Atlantic to Dakar, Canary Islands, etc., and then to Morocco, Spain, and Portugal. I can no longer remember what cruise line it was (definitely one of the major ones), but it was incredibly cheap, no catches, and equivalent in service/experience to any other cruise you would take on one of their 'normal' routes -- the main advantage was the unusual itinerary and the very low cost.
Also many years ago my parents did a few across the Atlantic to Europe as well -- also no problems.
So I say go for it if you like the itinerary and price -- they really can be a barain!!