What Is the Primary Risk for Liquidity Providers in a Stableswap Pool If One Stablecoin De-Pegs Severely?

The primary risk is that the pool will be drained of the stablecoin that maintains its peg, leaving the liquidity provider holding a disproportionately large amount of the severely de-pegged, less valuable stablecoin. Since the stableswap curve is very flat near the peg, arbitrageurs can exploit the price difference until the pool is saturated with the cheap asset.

This is a form of impermanent loss, but with the high risk of permanent capital impairment.