What Jerry says is true and bears repeating.

It is much easier to swap a spindle for a router on a bolt together kit machine than on a factory assembled one. Most of the factory built machines have the VFD installed inside the control cabinet (preferred) and use control inputs to determine a VFD fault, spindle stopped and up to speed along with speed control. The wiring of these safety features, most of which are left off the kits due to simplicity, may be beyond the abilities of many users.

Also... in many cases control systems that are wired for routers are 115VAC and those wired for spindles are 230VAC. Most users are not comfortable making the numerous line voltage changes required to change a custom built system from 115v to 230v and hardwire in a contactor and VFD. The kits tend to "fudge" the safety requirement by by routing VFD power thru a contactor with cords, but it is never switched by a load break switch, leaving live 230v power inside the box, even after a load break door safety switch is turned off.

My opinion is that if you wish to properly install a spindle and VFD, it is a much more difficult task when you install them according to NEC and industrial safety standards. Much, much more difficult than hanging the VFD in open air and connecting only power (via extension cords) and a spindle forward wire.