Assuming no Ribbis problems, of course.
Paying back one Gemach with another (“Gilgul Chovot”) is more expensive. There are exchange rates to deal with because some loan Gemachim work with another currency. There are the travel expenses. There is time lost when one could be earning money. And there is the natural tendency to use some of the money for oneself when there is “extra” left over between loans.
So I heard in the name of an expert who helps debtors.