You can make the deepest 1/4" mortises with a mortiser or mortising attachment for a drill press. That's only easy if you are mortising into the side of something. Of course the old standby method is to go as deep as you can with the spiral and then, using the sides of that mortise for a guide, deepen the hole with a drill bit and then chisel the rest of the waste out.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.