There is no wood that can resist sharp carbide bit at the proper feeds and speeds and using a technique exactly for it. Its art .

Fretwork is difficult. Meaning that needs planning. What Eddy says is one way. Depending on thickness desired, material and size there are other ways:

-use V bit and route the design leaving 1mm of material bellow. Flip and surface or just sand it.
-use waste board and screw the material where possible
-use very thin double sided sheet tape.
-mount 2-5w laser diode and just burn it if thin veneer
-make your DIY Swivel Drag Knife Cutter