They can be produced quickly and with rather low costs. Changes of the profile or the outline can be planned on a computer and implemented with the help of a computerized mill.
The advantages for the sailor should be a low price, a greater variety of designs and toughness towards damages. Good manoeuvre oriented fins quite often are constructed this way.
But, there is a list of disadvantages which are weighing heavier the longer the fin is. Milled fins are rather heavy. Their speed of response is not very high. If you want an optimum twist-flex reaction you might have to change the ideal profile and if you want the perfect profile, this might be at the expense of the twist-flex reaction that you expect. This means that a longer milled fin mostly has to be the result of a compromise.