I will argue that the kettlebell, when used in hardstyle philosophy, is the most multifaceted training tool available to you.
For simplicity sake let's compare it to the dumbbell:
Anyone that compares/uses the dumbbell as a replacement tool for the kettlebell needs to get an education....or at least learn or try training with a kettlebell APPROPRIATELY. There is no dumbbell simulation of a swing, squat, press, clean, snatch, or Turkish Get Up that compares to the benefits of these exercises using a kettlebell, the real gym in your hand.
As with most training tools, the dumbbell does have a purpose. It can be used to simulate the aforementioned movements (inefficiently when compared to the kettlebell) and is the safer tool when performing specific biomechanical movements. When including safety into the equation, the dumbbell may trump the kettlebell when pressing from the supine position. Additionally, the dumbbell is ideal when training shoulder specific range of motion exercises that may be beneficial to the golfer, baseball player, and rehab patient. Furthermore, the dumbbell is the safer device when performing ground based rows (the renegade row), albeit these rows are not in the same universe as the superior benefits of the ground based row using kettlebells.
A little about misuse:
As with most industries of business, personal trainers will take the kettlebell and invent silly movement variations of the swing, press and God forbid, the Turkish Get Up without the practical experience of learning the movement before you apply the external resistance. One of my favorites is the trainer that coaches their client to "get up any way you can" when performing a TGU, instead of following the protocol of the exercise. To quote my favorite Master RKC, "you want to feel the pothole in the road with the TGU".
If you don't know how the use the tool leave it in the toolbox or find someone that can teach you how to use it....APPROPRIATELY.
Have a great weekend. I am off with my 1 kettlebell and my bodyweight to perform snatches, thrusters, push ups, squats, burpees, sit ups, clean and press, 1 arm row and swing.......MULTIFACETED.