Thomas Sanladerer just published a video comaring 3D Printers (MP3DP ?) and CNC machines (MPCNC ?) So, this could interest all of us 
Sounds like he has more CNC based vids coming soon. Interesting to hear other points of view for sure. He had a bit of a rough go of it the first few tries, looks like he is getting the hang of it now.
Yeah, 3D printers are not really the best CNC mills, but they can work fine.
I managed to convert my Delta 3D printer to CNC milling a few years ago, it worked very well despite being the total opposite of rigid. I was able to mill wood and some very hard composite stuff, never tried aluminum but Iām pretty confident this would have worked too.
In my opinion, if anyone wants to build a machine that can do both applications, he should go for a CNC milling machine at first. Converting a CNC mill to a 3D printer is quite easy and it should work pretty well. The only issues will be printing speed due to the huge increase in mass, as well as Z height.