Actually, they are quite prevalent around here. They just don't measure up in popularity to the sharpmaker, but the sharpmaker doesn't measure up in envy potential to the edgepro which is the ultimate in guided systems. From what I gather around here several professionals use the edgepro in their business.
Varying angles are inherent with a guide system, but that is the trade off for perfect bevels. You can't change physics, but you can even the efects. For instance my knife is about four inches long and it has a pretty even taper from heel to tip. I set my bevel with my clamp right in the middle of my knife and that produced a 33 degree angle at the heel and a 35 degree angle at the center, and about a 37 degree angle at the tip. Had I known this when I set it, I would have cheated back to the heel till my heel and tip matched. This still wouldn't have been perfect, but much closer to even. The center still would have been a higher angle. These effects are greater with more taper, and also greater with a longer knife. These two things come into play to change the distance from the guide point or pivot point.
One thing that minimizes this effect is how long the guide point or pivot point is from the point of the clamp. I haven't seen the edgepro in person, but I am betting that the clamp point is much farther away from the guides.
Also, in theory, If I had more clamping room, I could turn my clamp to face the direction of my taper and the taper wouldn't come into play, however my aligner only grabs about 1/8" of knife material. This might also be different in the edgepro.
Don't sweat the lack of response on the lansky systems. More around here than will admit got started with one of these but have moved on to something else. They aren't bad tools, and can be used to put the keenest of edges if you use them properly. The problem is that you don't gain any skill at freehanding with these systems.
By the way, dmt's aligner works perfectly on a bench stone. All you have to do is put some tape on the outside of the guide to keep it from wearing down. I rebeveled my zdp blade from about 35degrees down to 27 degrees with my aligner and an 8 inch xx coarse dmt stone. The bevel line is perfect, but once again I have a varying bevel. 25 degrees at the heel 27 at the center and about 29 at the tip. Exactly 8 degrees lower than my primary bevel at these points.