The dark band between the two images may fool auto-exposure systems. Beamsplitters introduce keystone distortion because the outer mirrors are not exactly at a 45 degree angle. This is not a problem when the beamsplitter itself is used as a viewer, but can be if, for example, a print is made and viewed by free-viewing. Many users do not find this objectionable, but you might. It is important to keep the beamsplitter level with the camera.
Some models stay in place better than others.
If the front element of your SLR lens rotates as focus is adjusted, a beamsplitter may be a lot of trouble to use.