The Auger I used in the construction of my timber frame gazebo was very effective and only left me with two complaints:
1) It was challenging to keep it going plumb. (Crooked hole leads to crooked mortise)
2) It left you lopsided, because your right arm was always pulling down and your left arm was always pushing up.
This old time mortising machine is a bit like a hand cranked drill press.
The frame solves the issue of keeping the auger plumb to the surface of the beam. The hand cranks solve the issue of keeping both arms working in the same way so you don’t end up physiologically lopsided.
This video offers another view of it.
The modern counterpart to this machine would be something like this.
A drill mounted in a drill press/guide with a Forstner bit. This allows for creating holes for the mortise that are plumb and have a fixed depth (set by the guide). The two nice features of using the Forstner bit are that the holes can overlap (resulting in less waste that needs to be pared out) and the bottoms are flat.
This Makita cordless drill has plenty of power and can remove most of the waste for the mortise pretty quickly. For some mortises though the standard Forstner bit is not long enough. You can either buy longer Forstner bits or you can buy an extension. My suggestion is to go with the extension, it is a little more hassle, but much less expensive.