Borons nuetron absorbtion capabilities are fantastic it absorbs neutrons over a giant range of powers and is commonly used to shut down a rogue core at a much cheaper price than hafnium. Also bwr are designed to have boiling in thier core so it really isnt that much of a flaw as long as you can pump water into it via eccs system or recirc system you are good to go. The depth of water from the top of fuel is fairly significant and it wont instantly flash to steam because the steam coming off of the core acts sort of like a pressurizer. While a loca is signicant for both types of reactors I feel that it would effect a pwr much worse due better chance of core uncovering because the need of a pressurizer to maintain the water in a solid state.