All viewfinders found on digicams have their pros and cons but a few of the pitfalls will be mentioned here.
Optical viewfinders suffer from parallax error. As they are not completely in line with the lens, they show a slightly different view than the actual image. This will not be a problem in images taken from a distance, but when capturing close-ups this is something to take into account.
Of course you could also use the LCD screen in these situations.
The LCD screen however uses an enormous amount of battery power and can drain batteries in under 30 minutes if used a lot. The same is true for electronic viewfinders.
A few other disadvantages of electronic viewfinders are that they show an overly bright view, which makes it hard to judge correct exposure of the scene. Also the view through electronic viewfinders is quite coarse, which makes it impossible to check finer details in an image such as whether someone's eyes are open or partially closed.
While no system is perfect, the general idea is that optical TTL viewfinders are best in most situations, since they don't suffer from parallax, don't use battery power and clearly show all the details in a scene.