Any vang system will prevent boom rise and resultant ''bagging'' and undue twisting off/flow separation at the head of the sail, etc. For ''heavy downwind'' conditions I still use a preventer - dont make the lazy mistake of substituing a good preventer, etc. with a rigid vang.
To my mind a rigid vang outperforms a tackle vang/topping lift when there is need to overcome the mass of a heavy boom pulling down on the leach such as in light air conditions when you need to lift the boom to add a bit of twist, etc. and in those conditions where the boat is deep in a wave trough and the main is perioidically shadowed by huge waves. On most boats (unfortunately) the topping lift is usually led to the mast base, and because its inconvenient (or too dangerous) to keep running back and forth to adjust, one simply doesnt adjust the topping lift in changing conditions. Im not one who reefs as I should; but, prefer to flatten and blade-out the sails when necessary (especially when driving into heavy chop, etc.) ... a rigid vang really helps as its usually so easily controlled from the cockpit. Another reason for my choice of a rigid vang is simplicity of control ... the rigid vand combines the actions of a tackle vang and the topping lift into one package - less rope ''salad'', faster adjustment, less work, ... hence safer etc. Nevertheless, on my boat I still use a boom gallows and I have to fully unload the rigid vang to do so.