End mills can be used in routers instead of spiral cutters, assuming you have the appropriate collet to make them fit your router arbor. If you have up-cut and down-cut bits, be sure you're protecting yourself from unexpected stock seize-ups.
You can also use them in your drill press for free handing mortising and slotting, but be sure to have hold-downs and fencing, particularly with up-cut bits.
A big advantage of end mills is that they are ground all the way to the center, so you can plunge with them. I still recommend keeping the workpiece moving at least a little bit to transfer the cutting force to the wings.
FWIW, I'm an old end-mill mortising guy. We had a mortising machine with draw collets for a complete range of end mill diameters, and we did almost all our mortising on that machine -- from picture frames to driveway gates. It is a valid way to go. Be aware that tool-steel bits will dull faster than carbide, so you'll have to anticipate your burn out rate based on the woods you're routing.