Increased mutations/mutation rate is not necessarily a downside. Mutations aren't inherently bad, they can be beneficial, and the vast majority actually have no effect whatsoever, as the majority of our DNA is never used. But yes increased mutations means increased chances of bad mutations happening even if it also means increased chances of good mutations happening. Anyhow, it may put evolutionary pressure for longer lifespans. Studies involving fruit flies, and selective breeding them as old as possible for successive generations, show that can be the case. And other studies involving natural selection show that high mutation rates are a good thing (ie they help species survive), when environmental change is high (but high mutation rates are detrimental if the environment remains stable).