My experience with routers tells me that they may be the perfect tool for shallower work such as housings
But for something like a mortise you will run into several problems.
The first being that the longer bits tend to cavatate esp. when fully extended
Most any long bit you find will be a core box with cutting edges parallel to the turning axis and with little cross sec perpendicular to the axis. This results in a tool that is chipping as opposed to slicing and which flexes when it meets resistance. (Yes I know that is the action of power planes but in their case the cutter is supported on both ends)
Another problem is that this type of bit is prone to heating up, which of course means it doesn’t cut as well and so heats up etc….
Lastly it will no clear at all which only adds to the above problems
Having sounded all those sour notes I would suggest some things:
One, look for a spiral cutting bit
Two: consider using the router to finish the mortise after have removed the bulk of material from the center by other means.
Three: don’t try any of this with less than a three horse router
Four: Keep the bit sharp and clean, scrape resin build up off the cutting edge and clean with acetone and touch the edge up with a diamond stone from time to time.
