I've been traveling to Argentina for 25 years and have never once been asked by immigration for an onward ticket. If you have a US/Canadian/European passport the likelihood of that happening is close to zip.
In Brazil, however, the situation is different. Brazil doesn't require visas for Europeans but has recently tightened rules for Spanish citizens (other countries will come too) because of increased illegal immigration. Brazil asks to see credit card statements showing credit limit availability as well as an onward ticket for entry. They are not doing this to everyone but the fact that they do it at all is higher risk.
If you want to 100% cover yourself make a dummy reservation and print it out from say, Buenos Aires to Montevideo and bring it with you.