Impermanent loss isn’t a real loss, it is a loss of opportunity, of what “could have been” if you had just held the assets in your wallet. The reason is because Balancer will automatically reduce the quantity of the asset that is increasing in price to maintain balance, whereas if it is in your wallet and you held the asset that is increasing just becomes a larger % of your wallet. For example if you provided liquidity to the LEND/WETH pool and LEND outperforms ETH by 100% you would make around 75% gains on Balancer and 100% if you just held it in your wallet. The impermanent loss is 25%, or the difference between the two scenarios. Of course you wouldn’t have lost 25%, but you just would have made 75% rather than 100%. Most people providing liquidity to balancer are looking for steady yield, not to speculate and maximize gains on spot price.
One of the only actual risks of loss would be contract risk where some sort of bug is exploited and funds are drained. Of all of the AMM’s, Balancer is one of the most trusted and their smart contracts are heavily audited, so they are my preferred AMM compared to the others.