I know of several retirees who were granted citizenship. But whether they relied on a pension for their income, I don't know. These were probably between 5 and 15 years ago, after 4 or more years of residency I'd guess. The minimum is 3 years for married couples or 5 years if you are single.
I didn't hear that they had any special trouble because they were retirees. Maybe important is that they applied in Punta del Este. One told me that they were advised that the process is easier, or maybe smoother, outside Montevideo. But also, they lived in Punta anyway.
I also know of several people, not retirees, who applied for citizenship and didn't get it, even one couple who applied using a high profile lawyer. They were never exactly told 'no', but at each appointment they were told that they needed to provide more documentation, and it never ended. These were in Montevideo btw.