Incentive decay and reward token price
A high rate funded by token emissions falls as more capital arrives, because the same emission is split further. Simultaneously, farmers selling the reward token push its price down, which reduces the rate again in dollar terms.
The quoted number on day one is rarely the number anyone receives by week four.
Impermanent loss on the position itself
Most farms require a liquidity position, which carries divergence loss. On a volatile pair this can exceed the entire reward, and it is denominated separately from the yield so it never appears in the rate.
Correlated pairs reduce it; pairing a volatile token against a stablecoin maximises it.
Contract and oracle risk
You are interacting with the pool contract, the farm contract, often a vault wrapper, and whatever oracle they rely on. Each is a separate piece of code that can fail, and complexity compounds rather than averages.
Audits reduce the chance of certain bugs within a scope and do not make this risk zero.
Stacked positions multiply this. Depositing into a vault that deposits into a protocol that borrows from a third means three contracts must all behave, and a failure anywhere unwinds the whole position. The advertised rate rarely mentions how many layers produced it.
Exit liquidity and gas drag
Rewards are only worth the price at which you can actually sell them. A thin reward token means your exit moves the price against you, sometimes substantially.
Frequent compounding on an expensive chain also quietly consumes a meaningful share of the yield. FBT Swap shows farm and yield data with its source and never presents any of it as guaranteed income.