it will always be detrimental to accuracy. also the efficiency suffers as the pellet has to be sized to the bore using energy from the air blast, rather than your thumb. It also won't seal as well in the tap, and finally, due to variations in pellet sizes, many pellets will sit further down in the tap, increasing lost volume and further reducing efficiency. last but not least, the tap itself may leak air a little.

For fun shooting at 20-30 yards, it's fine, and very safe. But for optimal efficiency/accuracy, it's actually one of the worst systems.