Not that accuracy is the problem, but speed. I know that a beam borer with a good sharp bit will bore surprisingly quickly through quality timber, but I really don't know that I could match the speed of a German mortiser. That may be because I don't know all the "tricks". What do you find to be your most expediting techniques with a beam borer?
And yes, there is no comparison between the speed of a cross grain mortiser and the Makita. I would say the Germans are two to three times as fast. I still like my Makita to take to jobsites though.
The boring machine bores to the exact depth needed, with a flat bottom. The boring takes the bulk of the time, but cleanup can be very fast. Like 30 seconds to get the triangles on the sides and then one straight down shot on each end to finish. Also, you don't have to adjust your design to accommodate the mortiser. Like Will said, you can go an extra bit to eliminate the curve on the bottom, but usually the machine won't bore that deep. If you have a 1" housing, then you only get 4 3/4" of actual mortise depth, and it takes a lot longer to clean out that curve to get it than it does to clean up a boring machine mortise. One place a mortiser shines is with through mortises. If you can punch through from both sides, then there is almost no cleanup.
There are a couple of things I like about the mortiser. Ours will bore very straight ends on the mortise, and the far side line is also usually very crisp. The near side gets blown out, and is usually tapered at the bottom needing cleanup. I've experimented with running the chain loose, which helps, but is not consistent.
Another gripe of mine regarding power tools is what I call the power tool mentality, which is the compulsion to have every part of the frame on horses at the same time. This drives me nuts, especially is someone puts two timbers on the same horse. Unless you have an acre and a half, then you are constantly fighting for room to work. I'm an axe man, and it really sucks to have to reach across a timber to get to one far enough away to chop a housing. There's another pet peave, the "death by a thousand cuts", as I call it, where someone does a darn good display of singlehandedly supporting the saw blade industry by trying to kerf a tenon or housing to death. I can rough chop a housing or tenon with an axe faster than you can kerf with a circ saw.
Of course this is just my experience and opinion. I know it doesn't work for everyone. I know some people that just aren't comfortable working with an axe to close tolerances. In my personal work, there is no way I could afford to tool up with power tools. I cut small frames, and they wouldn't gross enough to buy a mortiser, and I don't want to think about how many I would have to cut just to break even, never mind actually become profitable with one.
