The annuity rate — the percentage of your annuitised corpus paid out each year — is determined at the time you purchase the annuity, not when you originally contributed to NPS, which means it depends on prevailing interest rates and the specific insurer's pricing at your retirement date, something impossible to predict decades in advance.
Most annuities pay a fixed amount for life with no automatic inflation adjustment, which means the real purchasing power of that pension quietly erodes every year you receive it — a pension that feels comfortable at 60 may feel considerably tighter at 80 purely due to accumulated inflation.
Some annuity variants offer a return of purchase price to your nominee on death, or joint-life coverage for a spouse, typically at a lower monthly payout in exchange for that added protection. Reading the specific annuity option's terms carefully at the time of purchase — rather than assuming all annuities work the same way — is worth the extra time given how long you'll likely depend on the income.