We have a Ultimaker 2+ Extended that we use to prototype alum extrusion shapes constantly. Here are the things I'm not sure how you would get around.
1) Printer head collisions with the aluminum part. While I'm sure there is some way to get around this if you write your own gcode to control the printer, normal slicer applications don't have any (that i know of) provisions for avoiding hitting stuff that it didn't print. I would suggest finding ways to design the part so the aluminum sleeve can slide into the body post printing.
2) Adhesion is going to vary by the material your printing with. ABS is probably ideal in terms of strength, PSI rating etc. Its also a pain in the ass to print. Roughing up materials might help. Once you get into exotic materials such as printable carbon fiber stuff I have no idea.
Past these two things, its super useful. I have printed up eye covers for guns, triggers etc for paintball stuff. Work great, super light weight. If you have more questions or want to run a design past us to see if we see anything wrong with it feel free to email me.