Since the kettlebell boom in English speaking America in recent times in early 2000s, many other English speaking first world countries have also been partaking in the "kettlebell revolution".
A lot of the marketing materials give the impression that and say things along these lines:
-Kettlebells are better than cardio.
-Kettlebells are better than conventional weight training / lifting.
-Kettlebells are better than conventional weights.
There can be other claims but to keep things simple, let's just use these broad categories of statements as examples.
Now, let me burst your bubble. These statements are not fair comparisons. They are comparing apples and oranges. In some sense they can be true, but as in all things, there is no hard and fast rule, aka "it depends". Allow me to explore in greater detail the claims made by these statements and their implications.
But before i go further, you need to have a background knowledge of: the Time is more important than the Technique, the Technique is more important than the Tool.
Kettlebells are better than cardio
This is an illogical statement. Kettlebell is a Tool, cardio is a Time / Protocol. Can you say "dumbbells are better than cardio" or "barbells are better than cardio"? It doesn't make sense.
Yes i know that it refers to "kettlebell training" rather than the equipment itself. But still "kettlebell training" is a very vague statement. Just like the barbell can be used for Bodybuilding, Powerlifting, Weightlifting and many other purposes, the kettlebell can be used for these same purposes.
"Kettlebell training" makes as much sense as "barbell training" or "dumbbell training". It does not give you the slightest hint of the fitness goal(s) being trained for nor the Time / Protocol of the training. The same Tools can be used for fat loss, muscle gain, strength & conditioning and many other purposes. Any of them can be used for 5x5, 3x10, 3x5, 10x3 etc. If you like, kettlebells can also be used for cardio or other nonsensical training Protocols, like "toning", "shaping", "slimming", etc.
More on cardio
The majority of people's idea of "cardio" is "long slow distance (LSD) cardio", aka "steady state cardio".
While there is nothing wrong with LSD if it fits your fitness goals (e.g. to run a marathon), if you are looking for efficient fat loss, it is not that efficient. There are much better suited training Protocols for fat loss, like High Intensity Interval Training (HIIT).
The problem is not that LSD is not good. The problem is that people do not know what Protocols to use for their fitness goals. Blame it on improper programming, not the Protocol. The body only knows how to adapt to the demands imposed upon it by you (SAID). You choose the Protocol based on your fitness goals.
Kettlebells are better than conventional weight lifting
Again, same point like the one above, an illogical statement. Kettlebell is a Tool, weight lifting (whether conventional or not is not the matter) is a Technique.
I do agree that the "classic techniques" of kettlebell lifting (eg. swing, clean, snatch) differ from barbell or dumbbell based systems, but beside these, both kettlebells and dumbbells / barbells can be used for the same exercises (eg. turkish get up, any variations of presses and other pushes, rows etc.).
In other words, you can use dumbbells for "kettlebell lifts" or kettlebells for "conventional weight training". You can even use kettlebells for curls if you like.
Granted the Technique varies to a degree when using different Tools. But if you are not a professional athlete who needs to perform with that specific Tool, then it really does not matter so much as doing the Exercises specific to your needs.
More on conventional weight training
What the majority of people know as "conventional weight training" is isolation exercises, possibly an overemphasis of the bench press and using machines, which are "non-functional".
Most people cringe when told about the real lifts: squats hurt your knees, deadlifts hurt your back, overhead lifting hurt your shoulders. But these lifts the real thing about "conventional weight training". Of course there are many more good exercises in "conventional weight training" and they are not to be shunned. It is because of some "rehab gurus" or some misinformed "fitness experts" that these ideas get into the mainstream and giving real weight training a bad rep.
Couple that with improper Protocols and other programming variables and the public will be tricked into believing that "conventional weight training" does not give them the results they want.
Blame it on misinformation, not on "conventional weight training". This is NOT real "conventional weight training", this is a misrepresentation. This is improper weight training. There are many good "conventional weight training" systems and programs out there, only if the public care to find out about them. There is nothing bad nor wrong about "conventional weight training" that needs to be replaced with "kettlebell training".
Kettlebells are better than conventional weights
A weight is a weight. As long as it has mass and therefore exerts weight on the user, it has fulfilled it's purpose. Does it matter so much what shape it is to you? It does matter to a degree, but that is outside the scope of this discussion.
A tool is just a tool. A tool is useful for what it is intended for. It is not useful for what it is not intended for. You wouldn't use a screwdriver to drive a nail, would you?
Which mode of transport is better? Walking, bicycle, motorcycle, car, ship, plane? Neither. Each is better in its own way depending on what it is used for. You wouldn't walk halfway round the globe, would you? Neither would you take a plane to the shop round the corner. Yes you could (i.e. effective), but it is a stupid idea to do that (i.e. inefficient).
So whether the weight is shaped like a kettle, or it is simply a rock, a bag of sand, a bar etc. it does not matter so much. It depends more on the Exercises you are doing. Some Exercises are done better with a kettlebell than dumbbbell. Some the other way around.
The kettlebell is just a tool in the tool box. There is no magic in the kettlebell. As if owning one would instantly turn you into a superman. Same applies to barbells, dumbbells, Clubbells or any other fitness Tool in the market that have marketing materials that say that their Tool is better than this, better than that blah blah blah (think TRX, Purmotion, wobble boards, Bosu Ball, Swiss Ball etc.). As if without owning one, you are training your body less.
The bottom line to a good fitness program is well, programming. I say it again: even if you have nothing with but your bodyweight you can have good training. Check out Primal Stress if you haven't. It contains many unconventional bodyweight exercises.
Nevertheless, if you already have kettlebells and want to learn non-conventional ways to train with them, do check out TACFIT Kettlebell Spetsnaz.