Who do you consider Porteños? I always thought that it was someone born and raised in the city.
That being said. My wife is a porteña and I am surrounded by porteños. I think that doing business is difficult here for everyone. I can not tell you how many times we have stayed at home waiting for plumber, electrican, painter... to show up at a scheduled time. They never show and never call. So we repeat the process with someone new.
I can speak with a lot of experience with this topic and I do not agree that Porteños or Argentines are more dishonest than others. In my experience I have had much more negative experiences from foriegners in Argentina than any Argentinians.
I found local people fair and loyal in many instances and I have been very suprised at how decent they have been to me when doing business. I have heard so much negative stuff here about viveza criolla and cunning behaviour but those types are obvious a mile away for me.
They may not be as efficient as Europeans or North Americans but that does not constitute unethical behaviour. For me loyalty and common decency are the best qualities in man and there are many people here in Buenos Aires that live with these values.
In some cultures, e.g. in East Asia, South America and the Middle East, people tend to avoid saying no to requests.
They also intend to keep their promises (which are different in nature from e.g. a European or US promise), but as it sometimes turns out, they are unable to do so.
Try to live in e.g. Thailand, where people "even" smile, when they regret to have failed their promise, it's a culture thing.
You simply have to learn this and also slowly - guided by experience, yours and other's - hand pick the people who have the Western approch so they at least inform you that they are unable to fulfill their promise.