Well to play devil's advocate here.... I've done a lot of locks with spade bits, but cheap Chinese ones are the real abomination because they aren't sharp and they can't be sharpened. If you use a decent spade bit with a plain point as opposed to those ghastly Bosch jobbies with the threaded points (I mean one such as an Irwin or Bahco), it is possible to control the cut with a degree of accuracy. You do need to be competent with a drill, though, as the bit can exit through the side of the door if you are inattentive or just plain cack-handed. The same is true of 3D bits, except for the fact that they'll go out of line even faster. Forstner bits are also a really bad choice. For some reason the myth persists on thr 'net that you use them for any accurate hole. You do not. They are specifically designed to produce clean flat-bottomed shallow holes (i.e accurate in depth) for shallow recessed hardware such as flush bolts, sash pulleys, gib door hinges and Perko closer plates, etc. Used for deep drilling they are slow, tend to clog easily, can overheat and blue if run at too high a speed and tend to wander off line far too easily (on deep drillings) - presumably why someone used the design as a basis for the 3D bit. The cheap Chinese ones sold by the likes of Toolstation are in any case blunt to start with and just generally dreadful - decent Forstner aren't really cheap at all. So the best solution, as proffered by Dave, really is the good old auger bit (but not the hopped-up super stubby augers offered by Irwin, Armeg or others - they can be just as bad as the 3D bits in the way they behave) - ideally run at a low speed and withdrawn periodically to clear out the chips. Speed is the real killer when it comes to accuracy. Even more controllable if used in a brace.... Anyone wanting to use a router will need deep pockets: the Trend jig isn't cheap, it really needs a 1/2in plunge router with 1800 watts or more of power and the deep pocket cutters don't come cheap, either. If you aren't a die hard router user the thought of using a cutter which projects 70mm or more out of the bottom of the router should also give you the willies. So one for the trade lads doing multiples I'd say although deeper locks still require a bit of extra drilling with a spade bit and cleaning out with a chisel for that last 5 to 10mm of depth
As to the ends of the mortise some modern locks let you leave the mortise ends round. If not you can square them out with a chisel. I have a couple of registered mortise chisels at 5/8in and 3/4in to do this as most locks seem to be in those sizes and they really can clean out the ends of a mortise with 3 or 4 blows of a hammer - at least in chipboard core fire doors. A DIYer might find a 3/8in or so bevel edge chisel in 2 or 3 passes just as useful not to mention cheaper. Not as accurate or fast, though, however absolute neatness doesn't matter too much for the lock body mortise ends because after all they will never be seen
