An external monitor is an excellent alternative - or even adjunct - to the optical viewfinder options for SLR cameras, and have both plus points and minuses:
- they are easier to work with in many situations; you don't have to have your eye right behind the back of the camera while shooting
- extra features like screen markers, false color and peaking can be useful
- BUT they are large (and require some kind of mount or rig to attach it to)
- and the HDMI port on most SLRs isn't the securest way to connect a monitor in the field.