Kettlebell training has become popular recently due to a number of reasons: the Russians made great use of it for decades, its proponents claim its ability to build great amounts of strength and endurance, it's cheap, it's mobile, and it's compact. The list goes on. It seems like kettlebell training is the perfect approach to strength and endurance training, general fitness, and weight loss. However, the reliance on kettlebell training is a misunderstanding of how strength and endurance are acquired and how fat is reduced.
Specific muscles are not sensitive to different movements. Muscles are sensitive to one input: force. The muscle is stressed when it has a force imposed on it. When the stress is high enough, the muscle is overloaded. After overload, if the muscle is fueled adequately and allowed to rest, the muscle grows stronger. This is Selye's Generalized Adaptation Syndrome applied to muscles for building strength. If the same stress is applied to the muscle shortly after it is adapted, it will not overload the muscle. This is the same concept as a callous, where skin will get calloused, but the same work (with an axe or saw, for example) will not make the callous larger. It would require more work, such as a greater amount of wood cut. The same is true of strength training; greater force (weight) is required to drive development.
So, will starting kettlebell training build strength? For someone weak, of course it will. A weak individual will build strength from doing unweighted lunges or swimming, because any amount of new stress on the muscles will make a weak individual stronger. This doesn't make swimming an optimal strength training exercise. The kettlebell is, of course, able to stress a weak muscle and overload the muscle to allow it to recover and then grow stronger. An untrained individual may make strength gains using a kettlebell for a week or a month, but how much longer? A year? Five years? More importantly, can someone use a kettlebell to achieve the equivalent strength of a 500lb squat? No, it cannot.
Generally, the way proponents of kettlebell training will accommodate for this plateau is by doing three things. The first approach is to buy a heavier kettlebell. The detriment of buying another kettlebell is obvious: cost. A 53lb kettlebell would cost about $80. The second option is to use a different movement that will allow the same weight to stress a muscle more. Holding the kettlebell in a 'steering wheel' manner is intuitively more stress on the arms than holding the kettlebell near the chest. Both of these approaches will extend the strength increases for a time, but again, how long? Certainly not for longer than a few months even if the program is designed well.
The third method proponents of kettlebells use to attempt to extend strength is to increase the number of reps. This does not work to increase strength. It is intuitive to think that if someone can only do five kettlebell swings at 53lb, then they are weaker than someone who can only do ten swings, and this person is weaker than the person that can do one hundred swings; however, this extrapolation does not work. The aerobic energy system (oxygen required) is used when muscles expend energy for longer than roughly two minutes. The anaerobic energy system (no oxygen required) is used in the first two minutes of energy expenditure. The phosphagen system (a subset of the anaerobic system) is used in the first five seconds. This system will generate the highest amount of contractile force, and by extension, this system is what will best overload the muscle to produce adaptation. 200 kettlebell swings accomplishes this no better than 20 kettlebell swings. 200 kettlebell swings will certainly work the aerobic and glycolytic energy systems, but this is not optimal for strength development.
If the kettlebell, while attractive at face value, is suboptimal, then what is the most optimal tool for strength development? The barbell. It can be used to perform effective movements for strength training: the squat, the press, the bench press, the clean, and the deadlift. These are effective strength exercises, because they work many muscles at one time over a long range of motion. The human body works as a system, not a series of components. The human body adapts effectively when it is stressed as a system. A single squat work out (three sets of five reps) will work more muscles than a kettlebell workout done in the same amount of time.
In addition, the barbell can be loaded incrementally, by one pound if needed. Even if kettlebells were sold in one pound increments, who wants to buy a new kettlebell every time they make a one pound increase in strength? Barbells can also be loaded to any reasonable amount of weight, more than anyone would need in the period of their lifetime. This can easily be accomplished with the plates that load the barbell. Progress can be made with barbells for decades, not just months, and barbell training works on weak and strong trainees alike. A firefighter that can pick up a 250lb co-worker will be doing the same exercises that a 70 year old grandmother that can barely pick up a gallon of milk will be doing to get stronger.
A trainee doesn't need to be doing 100 kettlebell swings for time, 53lb turkish get ups, or 20 goblet squats. A trainee needs to learn how to barbell squat, put a bar on their back, and get strong. This will not be as convenient or cost as little as a single kettlebell. However, it won't be much more expensive or inconvenient, but it will make a trainee much stronger.