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While they are a very easy and popular method of dosing a pool with Chlorine they have several drawbacks when compared to automatic dosing with liquid Chlorine. 

Aside from the fact that one has to remember to dose the pool, the main issue with tablets and granulate chlorine is that of getting the correct dose. Typically there is either insufficient Chlorine present to properly sanitise the pool or far too much! 

Because Chlorine is only effective if the pH value of the water is between 7.2 and 7.4, when dosing manually with tablets or granulates it is important to ensure the pH is also maintained correctly. With manual dosing of both pH and Chlorine there is even more chance of getting the dose and therefore the pool water incorrectly balanced for swimming. 

Tablets or granulates nearly always contain cyanuric acid and are often described as stabilised Chlorine. Stabilised Chlorine contains Cyanuric Acid which is added to the tablets and granulates to ensure the chlorine is not broken down too quickly by ultraviolet sun rays. However, Cyanuric Acid does not disappear from the water, so over time it builds in strength and once it reaches a concentration of above 70mg/L it effectively prevents any added Chlorine from working. At this point, there is no alternative but to empty the pool or hot tub and start again with fresh water, not an ideal situation if you are on a water meter!