In addition to Moz's answer, with which I agree, a stem riser can affect the handling of a bike, and it's safety. In most cases, they are fine and perfectly safe. Any thing like this can be carried too far, however, and your frame is designed with particular limits on the amount of leverage placed on any part of the frame. There are always extra margins built in for safety, but carrying any kind of modification to the design of a frame TOO far can result in damage.
Also, too large a change can, depending on body proportion, put you in position that will pull you too far toward the rear of the frame. That change of balance can make the bike steer "skittishly" or "lightly", because there is not a balanced proportion of the weight on the front wheel (which handles your steering).
Meaning that a 1"--3" riser is probably fine, but installing a 10" riser is probably not.
Given funds and opportunity, you are in the long run better off getting a frame that is fit correctly, or a style of bicycle better suited to your needs. IMHO.