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Absence of friction It is generally known that the negative part of the rep (this is the second part of the rep where you let the weight down) is more effective than the first part. Machines always have mechanisms with a certain amount of friction. The friction works against you when you lift, and lessens the power when you let the weight down, doing exactly the opposite of what you would like.

Symmetry

When an exercise is done correctly, both sides of the body are treated the same way. When using free weights, it directly shows when you are training unevenly, as one side would be up higher or quicker. When using machines with fixed bars and everything, these things are not that noticeable.

Disadvantages of free weights are, of course, that they are less safe and require more training experience to use.

Advantages of free weights:
1) low cost of equipment
2) similarity to most daily activities and other sports
3) variety of exercises and movements possible
4) evidence of improvement is easily recognized
5) easy accessibility
6) more likely to develop symmetry
7) stabilizing muscles, co-contracting muscles are strengthened in addition to the main muscle worked

Disadvantages of free weights:
1) inconsistent matching of resistive forces and muscular forces throughout the movement.
2) In some exercises, it's not possible to train through a full range of motion.

With Isokinetic machines such as the popular Lifecircuits, there is a constant movement speed and the amount of muscle force applied determines the amount of resistive force encountered. Therefore, a greater muscle force produces a greater resistive force, and vice-versa. Most electronic and hydraulic machines have resistance only during the exertion of the muscle contraction (the "up" phase). Of course, there are some exceptions.

Advantages:
1) regulation of speed for a more controlled movement
2) varied and accommodating resistance forces
3) detailed performance feedback

Disadvantages:
1) equipment cost is high therefore, gyms tend to have fewer of them
2) inconsistent regulation of force
3) lack of eccentric muscle contraction (the "down" phase)