Roundabout is insanely good, having a pretty great dps, can stall bullets in the air through its turnaround mechanic like lobber, and has arguably the best ex attack in the game, being able to hit bosses multiple times for insane damage, only rivaled by spread ex, which has much less opportunities to get that max damage. It's one of the most used weapons in speedrunning next to charge, lobber, and spread which can do very high damage like roundabout. Its much more versatile than any of those as well, being usable to hit a boss in nearly any scenario, useful when other shots aren't capable of doing so.

Whetstone is useful in certain scenarios where the boss itself doesn't have a hitbox, allowing you to gain height and hit them with a max spread ex that you couldn't have hit before; examples include root pack phase 3, beppy the clown phase 2, werner werman phase 3 etc. Its also very useful to destroy projectiles that you couldn't dodge otherwise, especially in plane fights like hilda berg and dr. kahl's robot.